refactor(webserver): extract SPA dist serving to the frontend-static fallback seat

The webserver's built-in static dist serving becomes a single-owner fallback
seat (registerFallback/applyIndexTaps); the SPA server moves to the new
@deepseek-ai/dsh-frontend-static plugin so the composing application owns its
dist as composition, not carrier config. distIndex leaves the webserver
schema; unclaimed fallback answers 404.

packages/host/webserver/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/host/webserver/README.md
-README.md: 196f350d87c5322cd3e9cda6e40587d35acd08c4
-README.zh.md: 0ae0470eab0aae2f6b539404621c611d95827977
+README.md: b6dccf2f81c9e2f0b9f53264eafe724edb560f07
+README.zh.md: dbfe420013ed67c48e47048341f020864aeef16a

packages/host/webserver/README.md

@@ -2,11 +2,11 @@
 
 English | [中文](README.zh.md)
 
-Web HTTP and upgrade-route registration plugin (default-exported `HttpServerService`, config `{host, port, distIndex}`): a `node:http` server that listens on activation and provides `ctx.httpServer`. `register(route)` adds a named `exact`/`prefix` HTTP route; `registerUpgrade(route)` adds an upgrade route for an exact pathname. A duplicate path within either table throws because route patterns are a composition-level contract and a collision is a misconfiguration; both methods return a disposer that removes the registration. `tapIndex(transform)` adds an index.html transform applied in registration order, `port` reads the listening port (the OS-assigned value when `port` is 0), and `host` reads the configured bind host (composition-time facts other plugins adapt to, e.g. the directory-picker chooser). HTTP match order is fixed: exact over the whole table, then longest prefix, then the static dist fallback with the locked semantics: traversal outside the dist root is 403, any miss falls back to `index.html` with HTTP 200 (SPA routing), unknown extensions ship as octet-stream, and non-GET/HEAD is 405. Upgrades match exactly and unmatched connections are closed; registration order carries no request-facing semantics.
+Web HTTP and upgrade-route registration plugin (default-exported `HttpServerService`, config `{host, port}`): a `node:http` server that listens on activation and provides `ctx.httpServer`. `register(route)` adds a named `exact`/`prefix` HTTP route; `registerUpgrade(route)` adds an upgrade route for an exact pathname. A duplicate path within either table throws because route patterns are a composition-level contract and a collision is a misconfiguration; both methods return a disposer that removes the registration. `registerFallback(handler)` claims the single fallback seat answering everything no named route matches — one owner only (a second claim throws; the SPA dist server [`dsh-frontend-static`](../frontend-static/README.md) is the shipped owner), 404 while unclaimed. `tapIndex(transform)` adds an index.html transform, and `applyIndexTaps(html)` runs a body through the registered transforms in order — the fallback owner calls it on every index response. `port` reads the listening port (the OS-assigned value when `port` is 0), and `host` reads the configured bind host (composition-time facts other plugins adapt to, e.g. the directory-picker chooser). HTTP match order is fixed: exact over the whole table, then longest prefix, then the fallback seat. Upgrades match exactly and unmatched connections are closed; registration order carries no request-facing semantics.
 
-The package knows no harness concepts: the `/api` HTTP bridge and downlink WebSockets are routes owned by the connection plugin, while plugin bundles and the HMR event stream are routes owned by the modules/hmr plugins. The upgrade handler owns the protocol handshake and connection contents; the webserver only delivers the raw socket and request. `host` accepts only `127.0.0.1` (default posture) and `0.0.0.0` (deliberate network exposure); `distIndex` is an assembly fact the composing app resolves and injects, never self-resolved (dist location is workspace knowledge of the app). Web (browser) shape only — Electron loads dist over `file://` and carries fetch over an IPC bridge, not this server. This package never prints; the URL line belongs to the shell.
+The package knows no harness concepts and serves no files: the `/api` HTTP bridge and downlink WebSockets are routes owned by the connection plugin, plugin bundles and the HMR event stream are routes owned by the modules/hmr plugins, and dist serving belongs to the fallback owner. The upgrade handler owns the protocol handshake and connection contents; the webserver only delivers the raw socket and request. `host` accepts only `127.0.0.1` (default posture) and `0.0.0.0` (deliberate network exposure). Web (browser) shape only — Electron loads dist over `file://` and carries fetch over an IPC bridge, not this server. This package never prints; the URL line belongs to the shell.
 
-A listen failure (EADDRINUSE…) throws out of activation and rejects Loader composition with the bind diagnostic; the failed candidate fiber is disposed. An HTTP request whose handling throws (a malformed %-escape hitting `decodeURIComponent`, a client dropping mid-body) is answered 400 — or the socket destroyed when headers are already out — and logged as a warning; it never exits the process. An upgrade-handler exception or upgraded-socket transport error is logged as a warning and destroys its socket. Disposal starts `close()` and `closeAllConnections()`, destroys every tracked upgraded socket, and returns only after the HTTP server and those sockets have closed.
+A listen failure (EADDRINUSE…) throws out of activation and rejects Loader composition with the bind diagnostic; the failed candidate fiber is disposed. An HTTP request whose handling throws (a fallback owner's `decodeURIComponent` on a malformed %-escape, a client dropping mid-body) is answered 400 — or the socket destroyed when headers are already out — and logged as a warning; it never exits the process. An upgrade-handler exception or upgraded-socket transport error is logged as a warning and destroys its socket. Disposal starts `close()` and `closeAllConnections()`, destroys every tracked upgraded socket, and returns only after the HTTP server and those sockets have closed.
 
 In development, the client-plugin registry synchronously captures each built bundle's stat baseline before it returns, then polls those baselines and re-hashes changed content. Each rescan stages its candidate table, graph, and watch map before publishing them, so a baseline failure preserves the prior graph. An immediate rebuild therefore cannot disappear into an asynchronously established watch baseline; a rename window marks the path dirty, retains the last successful baseline, and forces a re-hash when the bundle reappears even with identical metadata.
 
@@ -21,5 +21,4 @@ None; this package neither assembles nor sends a provider request.
 ## Known Limitations and Deferred Work
 
 - **No TLS, auth, or origin policy** — binding a non-loopback address exposes the server to that network; deployment hardening (or fronting it with a real reverse proxy) is deliberately out of scope for the dev-facing v1.
-- **The starter MIME table is minimal** — extensions beyond the vite-emitted set fall back to `application/octet-stream`; extend the table when an asset class actually ships.
 - **Socket options are fixed** — config selects the bind host and port, while backlog and other socket settings remain internal until a deployment needs them.

packages/host/webserver/README.zh.md

@@ -2,11 +2,11 @@
 
 [English](README.md) | 中文
 
-Web HTTP 与 upgrade route 注册插件（默认导出 `HttpServerService`，配置为 `{host, port, distIndex}`）：一个在激活时开始监听的 `node:http` 服务器，提供 `ctx.httpServer`。`register(route)` 添加具名的 `exact`／`prefix` HTTP route；`registerUpgrade(route)` 添加精确 pathname 的 upgrade route；同一张表内的重复路径会抛错，因为 route 模式是组合层契约，冲突即配置错误；两者返回的 disposer 都会移除注册。`tapIndex(transform)` 添加按注册顺序应用的 index.html 转换，`port` 读取正在监听的端口（当 `port` 为 0 时读取 OS 分配的值），`host` 读取配置的绑定宿主（这些是其他插件据以自适应的组合期事实，例如 directory-picker 选择器）。HTTP 匹配顺序固定不变：先在整张表中匹配精确 route，再匹配最长前缀，最后回退到静态 dist，并遵循固定语义：越出 dist 根目录的遍历返回 403，任何未命中项都以 HTTP 200 回退到 `index.html`（SPA 路由），未知扩展名按 octet-stream 提供，GET／HEAD 之外的方法返回 405。upgrade 只做精确匹配，未命中连接直接关闭；注册顺序不承载任何面向请求的语义。
+Web HTTP 与 upgrade route 注册插件（默认导出 `HttpServerService`，配置为 `{host, port}`）：一个在激活时开始监听的 `node:http` 服务器，提供 `ctx.httpServer`。`register(route)` 添加具名的 `exact`／`prefix` HTTP route；`registerUpgrade(route)` 添加精确 pathname 的 upgrade route；同一张表内的重复路径会抛错，因为 route 模式是组合层契约，冲突即配置错误；两者返回的 disposer 都会移除注册。`registerFallback(handler)` 认领唯一的回退席位，应答所有未被具名 route 命中的请求：只允许一个持有者（第二次认领会抛错；随附的持有者是 SPA dist 服务器 [`dsh-frontend-static`](../frontend-static/README.md)），席位未被认领时返回 404。`tapIndex(transform)` 添加一个 index.html 转换，`applyIndexTaps(html)` 按注册顺序对一段响应体运行已注册的转换：fallback 持有者在每次 index 响应时调用它。`port` 读取正在监听的端口（当 `port` 为 0 时读取 OS 分配的值），`host` 读取配置的绑定宿主（这些是其他插件据以自适应的组合期事实，例如 directory-picker 选择器）。HTTP 匹配顺序固定不变：先在整张表中匹配精确 route，再匹配最长前缀，最后交给回退席位。upgrade 只做精确匹配，未命中连接直接关闭；注册顺序不承载任何面向请求的语义。
 
-该包不了解任何 harness 概念：`/api` HTTP 桥接与下行 WebSocket 是 connection 插件的 route，插件 bundle 与 HMR（热模块替换）事件流则是 modules／hmr 插件的 route。upgrade handler 拥有协议握手与连接内容；webserver 只交付原始 socket 与 request。`host` 只接受 `127.0.0.1`（默认姿态）和 `0.0.0.0`（有意向网络开放）；`distIndex` 是由组合应用解析并注入的组装事实，绝不会自行解析，因为 dist 位置属于应用的工作区知识。该服务器只服务 Web（浏览器）形态；Electron 通过 `file://` 加载 dist，并经 IPC 桥接承载 fetch，而不使用本服务器。该包从不打印内容；URL 行属于 shell。
+该包不了解任何 harness 概念，也不提供任何文件服务：`/api` HTTP 桥接与下行 WebSocket 是 connection 插件的 route，插件 bundle 与 HMR（热模块替换）事件流是 modules／hmr 插件的 route，dist 服务则属于 fallback 持有者。upgrade handler 拥有协议握手与连接内容；webserver 只交付原始 socket 与 request。`host` 只接受 `127.0.0.1`（默认姿态）和 `0.0.0.0`（有意向网络开放）。该服务器只服务 Web（浏览器）形态；Electron 通过 `file://` 加载 dist，并经 IPC 桥接承载 fetch，而不使用本服务器。该包从不打印内容；URL 行属于 shell。
 
-监听失败（EADDRINUSE……）会从激活过程抛出，以 bind 诊断使 Loader 组合 reject；失败的候选 fiber 会被 dispose（资源释放）。处理 HTTP 请求时抛错（例如格式错误的百分号转义传入 `decodeURIComponent`，或客户端在请求体传输中途断开）时，服务器会响应 400；若响应头已经发出，则销毁 socket，并记录 warning，但绝不会退出进程。upgrade handler 抛错或升级 socket 出现传输错误时，会记录 warning 并销毁对应 socket。资源释放会启动 `close()` 与 `closeAllConnections()`，销毁所有受跟踪的升级 socket，并仅在 HTTP server 与这些 socket 均已关闭后返回。
+监听失败（EADDRINUSE……）会从激活过程抛出，以 bind 诊断使 Loader 组合 reject；失败的候选 fiber 会被 dispose（资源释放）。处理 HTTP 请求时抛错（例如 fallback 持有者的 `decodeURIComponent` 收到格式错误的百分号转义，或客户端在请求体传输中途断开）时，服务器会响应 400；若响应头已经发出，则销毁 socket，并记录 warning，但绝不会退出进程。upgrade handler 抛错或升级 socket 出现传输错误时，会记录 warning 并销毁对应 socket。资源释放会启动 `close()` 与 `closeAllConnections()`，销毁所有受跟踪的升级 socket，并仅在 HTTP server 与这些 socket 均已关闭后返回。
 
 在开发环境中，客户端插件注册表会在返回前同步捕获每个已构建 bundle 的 stat 基线，随后轮询这些基线，并在内容变化后重新计算哈希。每次重新扫描都会先暂存候选表、图和监听 map，再统一发布，因此基线失败会保留先前的图。这样，即时重建不会消失在异步建立的监听基线中；重命名窗口会把路径标记为脏，保留最近一次成功基线，并在 bundle 重新出现时强制重新计算哈希，即使其元数据完全相同也不例外。
 
@@ -21,5 +21,4 @@ Web HTTP 与 upgrade route 注册插件（默认导出 `HttpServerService`，配
 ## 已知限制与延期工作
 
 - **不提供 TLS、认证或来源策略**：绑定非回环地址会向对应网络公开服务器；面向部署的加固措施（或在前方放置真正的反向代理）有意不纳入面向开发环境的 v1。
-- **初始 MIME 表很精简**：Vite 输出集合以外的扩展名会回退到 `application/octet-stream`；实际发布新的资产类别时再扩展该表。
 - **Socket 选项固定不变**：配置只选择绑定宿主与端口；在具体部署产生需求前，backlog 和其他 socket 设置仍保持内部实现。

packages/host/webserver/src/index.ts

@@ -1,21 +1,19 @@
 /**
  * @deepseek-ai/dsh-host-webserver — Web route-registration plugin: a node:http
  * server plus the `httpServer` service (HTTP and upgrade route registries,
- * index transform taps, and static dist fallback). Knows no harness concepts;
- * feature plugins own every registered protocol. Web shape only — Electron
- * loads dist over file:// and carries fetch over an IPC bridge. This package
- * never prints: the URL line belongs to the shell.
+ * index transform taps, and the single fallback seat for everything no route
+ * claims). Knows no harness concepts and serves no files; the composing
+ * application's frontend plugin owns dist serving through the fallback seam.
+ * Web shape only — Electron loads dist over file:// and carries fetch over an
+ * IPC bridge. This package never prints: the URL line belongs to the shell.
  */
 
 import { createServer } from 'node:http'
 import type { IncomingMessage, ServerResponse, Server } from 'node:http'
-import { readFile } from 'node:fs/promises'
 import type { AddressInfo } from 'node:net'
 import type { Duplex } from 'node:stream'
-import { dirname } from 'node:path'
 import { Context, Service } from 'cordis'
 import z from 'schemastery'
-import { serveStatic } from './static.ts'
 
 declare module 'cordis' {
   interface Context {
@@ -43,28 +41,26 @@ export interface WebUpgradeRoute {
   handler: (req: IncomingMessage, socket: Duplex, head: Buffer) => void | Promise<void>
 }
 
-/** Gateway config: listen address plus the static dist anchor (injected by the composing app, never self-resolved). */
+/** Gateway config: the listen address. */
 export interface Config {
   /** Listen host; the two supported values are loopback and all-interfaces. */
   host: '127.0.0.1' | '0.0.0.0'
   /** Listen port; zero requests an OS-assigned port. */
   port: number
-  /** Absolute path of index.html inside the static root (dist location is workspace knowledge of the app). */
-  distIndex: string
 }
 
 /**
  * The web-shape HTTP carrier service. Activation listens immediately (route
  * registration order carries no request-facing semantics: named routes are
- * composed to be disjoint, and the static dist fallback answers anything not
- * yet claimed during the boot window). A listen failure throws out of init —
- * a FAILED fiber the boot's fail-loud sweep reports.
+ * composed to be disjoint, and the fallback seat answers anything not yet
+ * claimed during the boot window — 404 until its owner registers). A listen
+ * failure throws out of init — a FAILED fiber the boot's fail-loud sweep
+ * reports.
  */
 export class HttpServerService extends Service {
   static Config: z<Config> = z.object({
     host: z.union([z.const('127.0.0.1'), z.const('0.0.0.0')]).required(),
     port: z.natural().max(65535).required(),
-    distIndex: z.string().required(),
   })
 
   private readonly exact = new Map<string, WebRoute>()
@@ -72,15 +68,12 @@ export class HttpServerService extends Service {
   private readonly upgrades = new Map<string, WebUpgradeRoute>()
   private readonly upgradedSockets = new Set<Duplex>()
   private readonly indexTaps: ((html: string) => string)[] = []
-  private readonly distRoot: string
-  private readonly distIndex: string
+  private fallback: WebRoute['handler'] | undefined
   private server!: Server
   private listenedPort!: number
 
   constructor(ctx: Context, private config: Config) {
     super(ctx, 'httpServer')
-    this.distIndex = config.distIndex
-    this.distRoot = dirname(config.distIndex)
   }
 
   /** The listening port (the OS-assigned value when config.port is 0). */
@@ -123,8 +116,24 @@ export class HttpServerService extends Service {
   }
 
   /**
-   * Register an index.html transform, applied to every index response in
-   * registration order.
+   * Claim the fallback seat: the handler answering every request no named
+   * route matches (the SPA dist server in the shipped Web composition). One
+   * owner only — a second registration throws, because two fallbacks cannot
+   * compose.
+   * @param handler - owns the full response lifecycle of unmatched requests.
+   * @returns the disposer releasing the seat.
+   */
+  registerFallback(handler: WebRoute['handler']): () => void {
+    if (this.fallback !== undefined) {
+      throw new Error('webserver: fallback already registered')
+    }
+    this.fallback = handler
+    return () => { this.fallback = undefined }
+  }
+
+  /**
+   * Register an index.html transform, applied by the fallback owner to every
+   * index response ({@link applyIndexTaps}) in registration order.
    * @param transform - pure html-to-html function.
    * @returns the disposer removing the transform.
    */
@@ -147,14 +156,13 @@ export class HttpServerService extends Service {
         await route.handler(req, res)
         return
       }
-      // Static fallback keeps the pre-plugin semantics: non-GET/HEAD is 405,
-      // traversal 403, miss falls back to index.html 200 (SPA routing).
-      if (req.method !== 'GET' && req.method !== 'HEAD') {
-        res.writeHead(405)
+      const fallback = this.fallback
+      if (fallback === undefined) {
+        res.writeHead(404)
         res.end()
         return
       }
-      await serveStatic(decodeURIComponent(rawPath), res, this.distRoot, this.distIndex, () => this.renderIndex())
+      await fallback(req, res)
     }
     // Last-resort guard: handle() rejecting would otherwise be an unhandled
     // rejection killing the process on one malformed request (bad %-escape,
@@ -243,11 +251,16 @@ export class HttpServerService extends Service {
     return best
   }
 
-  /** Index body: dist index.html through the registered taps in order. */
-  private async renderIndex(): Promise<string> {
-    let html = await readFile(this.distIndex, 'utf8')
-    for (const transform of this.indexTaps) html = transform(html)
-    return html
+  /**
+   * Run an index.html body through the registered taps in registration order
+   * — called by the fallback owner on every index response it renders.
+   * @param html - the raw index.html body.
+   * @returns the transformed body.
+   */
+  applyIndexTaps(html: string): string {
+    let out = html
+    for (const transform of this.indexTaps) out = transform(out)
+    return out
   }
 }
 

packages/host/webserver/src/static.ts

@@ -1,60 +0,0 @@
-/**
- * Static file serving for the web shell: the starter MIME table and the
- * request handler with the semantics locked by the step1 acceptance list —
- * traversal outside the dist root is 403, any miss falls back to index.html
- * with HTTP 200 (SPA routing), unknown extensions ship as octet-stream.
- */
-
-import type { ServerResponse } from 'node:http'
-import { extname, join, normalize, resolve, sep } from 'node:path'
-import { readFile } from 'node:fs/promises'
-
-const MIME: Record<string, string> = {
-  '.html': 'text/html; charset=utf-8',
-  '.js': 'text/javascript; charset=utf-8',
-  '.css': 'text/css; charset=utf-8',
-  '.svg': 'image/svg+xml',
-  '.json': 'application/json',
-  '.map': 'application/json',
-}
-
-/**
- * Serve one GET/HEAD static request from the dist root.
- * @param pathname - decoded URL pathname of the request.
- * @param res - the node:http response to write.
- * @param distRoot - absolute dist root directory (resolved by the caller).
- * @param distIndex - absolute path of index.html inside distRoot.
- * @param renderIndex - when set, produces the index.html body (boot-manifest
- * injection) for `/` and every SPA fallback; undefined serves the file verbatim.
- */
-export async function serveStatic(
-  pathname: string, res: ServerResponse, distRoot: string, distIndex: string,
-  renderIndex?: () => Promise<string>,
-): Promise<void> {
-  const target = resolve(normalize(join(distRoot, pathname)))
-  // Traversal rejection: the target must be distRoot itself (`/`) or stay under
-  // it. `sep`, not '/': resolve() emits backslash paths on Windows, where a '/'
-  // suffix would reject every legitimate subpath as traversal.
-  if (target !== distRoot && !target.startsWith(distRoot + sep)) {
-    res.writeHead(403)
-    res.end()
-    return
-  }
-  const serveIndex = async (): Promise<void> => {
-    const body = renderIndex === undefined ? await readFile(distIndex) : await renderIndex()
-    res.writeHead(200, { 'content-type': MIME['.html'] })
-    res.end(body)
-  }
-  if (target === distRoot || target === distIndex) {
-    await serveIndex()
-    return
-  }
-  try {
-    const body = await readFile(target)
-    res.writeHead(200, { 'content-type': MIME[extname(target)] ?? 'application/octet-stream' })
-    res.end(body)
-  } catch {
-    // Miss (ENOENT/EISDIR) falls back to index.html with 200 (SPA routing).
-    await serveIndex()
-  }
-}

packages/host/webserver/tests/webserver.spec.ts

@@ -2,11 +2,10 @@
  * REAL-composition coverage: a test-only cordis.yml booted through the
  * vendored Loader mounts the webserver row, and every assertion observes the
  * user-visible HTTP surface of the running server (routing precedence, index
- * taps, static-fallback semantics, per-request error containment, teardown).
+ * taps, fallback-seat semantics, per-request error containment, teardown).
  */
 
 import { mkdtemp, rm, writeFile } from 'node:fs/promises'
-import { mkdir } from 'node:fs/promises'
 import { once } from 'node:events'
 import { connect } from 'node:net'
 import { tmpdir } from 'node:os'
@@ -28,21 +27,15 @@ afterEach(async () => {
   root = undefined
 })
 
-/** Write a dist fixture and a cordis.yml with one webserver row, then boot it through the real Loader. */
+/** Write a cordis.yml with one webserver row, then boot it through the real Loader. */
 async function loadComposition(port = 0): Promise<Context> {
   root = await mkdtemp(join(tmpdir(), 'dsh-webserver-loader-'))
-  const dist = join(root, 'dist')
-  await mkdir(dist)
-  const distIndex = join(dist, 'index.html')
-  await writeFile(distIndex, '<head></head><body>shell</body>')
-  await writeFile(join(dist, 'app.js'), 'export {}')
   const configPath = join(root, 'cordis.yml')
   await writeFile(configPath, [
     "- name: '@deepseek-ai/dsh-host-webserver'",
     '  config:',
     "    host: '127.0.0.1'",
     `    port: ${String(port)}`,
-    `    distIndex: '${distIndex}'`,
     '',
   ].join('\n'))
 
@@ -96,7 +89,7 @@ describe('real Loader composition', () => {
   // Real-Loader composition resolves workspace packages through tsx at test
   // time; first resolution after the host/client program split is slow enough
   // to trip the default 5s budget on cold caches.
-  it('serves registered routes, index taps, and the static fallback semantics', { timeout: 60_000 }, async () => {
+  it('serves registered routes, index taps, and the fallback-seat semantics', { timeout: 60_000 }, async () => {
     const loaded = await loadComposition()
     const unloaded = [...loaded.loader.entries()]
       .filter(entry => entry.fiber === undefined && !entry.disabled)
@@ -120,21 +113,24 @@ describe('real Loader composition', () => {
     expect(await request(port, '/api')).toMatchObject({ status: 200, body: 'API' })
     expect(await request(port, '/api/anything', { method: 'POST' })).toMatchObject({ status: 200, body: 'API' })
 
-    // Index taps apply in registration order on `/` and on the SPA fallback;
-    // the disposer removes the transform.
+    // Fallback seat: 404 while unclaimed; the owner answers everything no
+    // named route matches; index taps are the owner's to apply; the seat
+    // admits exactly one owner and the disposer releases it.
+    expect((await request(port, '/no/such/route')).status).toBe(404)
     const untap = server.tapIndex(html => html.replace('<head>', '<head><script>window.__T__=1</script>'))
-    expect((await request(port, '/')).body).toContain('__T__')
+    expect(server.applyIndexTaps('<head></head>')).toContain('__T__')
+    const releaseFallback = server.registerFallback((req, res) => {
+      // Decode like a real static server would — a malformed %-escape throws
+      // here, probing the webserver's per-request error containment.
+      decodeURIComponent(new URL(req.url ?? '/', 'http://x').pathname)
+      res.writeHead(200, { 'content-type': 'text/html' })
+      res.end(server.applyIndexTaps('<head></head><body>shell</body>'))
+    })
+    expect(() => server.registerFallback(() => {})).toThrow(/fallback already registered/)
     expect((await request(port, '/no/such/route')).body).toContain('__T__')
     untap()
-    expect((await request(port, '/')).body).not.toContain('__T__')
-
-    // Static fallback semantics: real asset served, traversal 403, non-GET/
-    // HEAD without a matching route 405.
-    expect(await request(port, '/app.js')).toMatchObject({ status: 200, body: 'export {}' })
-    await writeFile(join(root!, 'dist', 'app.js'), 'export const rebuilt = true')
-    expect(await request(port, '/app.js')).toMatchObject({ status: 200, body: 'export const rebuilt = true' })
-    expect((await request(port, '/..%2f..%2fetc%2fpasswd')).status).toBe(403)
-    expect((await request(port, '/nowhere', { method: 'POST' })).status).toBe(405)
+    expect((await request(port, '/no/such/route')).body).not.toContain('__T__')
+    expect((await request(port, '/no/such/route')).body).toContain('shell')
 
     // Per-request error containment: a malformed %-escape answers 400 and the
     // server keeps serving afterwards (no process-level failure path).
@@ -148,9 +144,14 @@ describe('real Loader composition', () => {
     const disposeOnce = server.register({ kind: 'exact', path: '/once', handler: (_req, res) => { res.writeHead(200); res.end('ONCE') } })
     expect(await request(port, '/once')).toMatchObject({ status: 200, body: 'ONCE' })
     disposeOnce()
-    expect((await request(port, '/once')).body).toContain('shell') // back to the SPA fallback
+    expect((await request(port, '/once')).body).toContain('shell') // back to the fallback owner
     expect(() => server.register({ kind: 'exact', path: '/once', handler: () => {} })).not.toThrow()
 
+    // Releasing the seat restores the unclaimed 404 and registrability.
+    releaseFallback()
+    expect((await request(port, '/no/such/route')).status).toBe(404)
+    expect(() => server.registerFallback(() => {})).not.toThrow()
+
     // Upgrade routes match exact pathnames, reject duplicate ownership, and
     // become registrable again after disposal. The accepted socket stays open
     // so the teardown assertion also covers upgraded-connection ownership.