fix(llm,settings): refuse post-disposal route replacement and teardown notifications

Two lifecycle holes the registry and the consumer helper left open.

`AdapterRegistrationHandle.replace` had no liveness guard: after the handle's
disposer ran, a replace put routes back into the registry with nothing left to
release them, so the adapter leaked permanently. `owned` being empty cannot
carry that fact, because `replace([])` is the legal empty-section state, so the
disposer records it explicitly.

`installSettingsSection`'s watcher lacked the guard its own disposer carries:
a stored change landing while the consumer unloads reached `onChange`, which
re-registers routes against a fiber whose resources are being released.

Also documents `withFileLock` in the atomic-write README (it claimed one
export), records the age-based lock takeover as a known limitation, and lists
ctx.settings and ctx.credentials in the architecture capability table.

packages/util/atomic-write/README.zh.md

@@ -7,14 +7,20 @@
 ## 接口面
 
 ```ts
-import { writeFileAtomic } from '@deepseek-ai/dsh-atomic-write'
+import { withFileLock, writeFileAtomic } from '@deepseek-ai/dsh-atomic-write'
 
 declare const text: string
+declare const render: (previous: string) => string
 
 await writeFileAtomic('/home/u/.dsh/settings.yaml', text, { mode: 0o600 })
+
+// Read-modify-write against the same file from several processes.
+await withFileLock('/home/u/.dsh/settings.yaml', async () => {
+  await writeFileAtomic('/home/u/.dsh/settings.yaml', render(text), { mode: 0o600 })
+})
 ```
 
-仅一个导出。契约按故障利用它的先后顺序列出：
+`writeFileAtomic` 提交一份已经渲染好的字符串。契约按故障利用它的先后顺序列出：
 
 - **独占创建临时文件**（`wx` + 随机后缀）：open 拒绝跟随预先埋在可猜测临时路径上的符号链接。
 - **全新 inode 携带 `mode` 走完 rename**：替换权限过宽的旧文件时直接收窄，不存在 chmod 竞态。`mode` 为必填，让权限决策始终可见于每个调用点（与所有新建 inode 一样受进程 umask 影响）。
@@ -22,6 +28,8 @@ await writeFileAtomic('/home/u/.dsh/settings.yaml', text, { mode: 0o600 })
 - **同目录兄弟文件**保证 rename 落在同一文件系统上，交换保持原子。
 - 自动创建父目录；任何失败都会移除临时文件并重新抛出该失败；读取方只会观察到旧内容或完整的新内容。
 
+`withFileLock` 跨进程串行化同一文件的写入方，服务于单靠原子提交无法保证安全的读-渲染-提交循环。锁是以 `wx` 创建的同目录 `<filename>.lock`，因此读取方从不参与竞争；等待方按指数退避，超时即失败而非无限阻塞。超过陈旧时限的锁被视为持有者已崩溃并被打破——其代价见[Known Limitations and Deferred Work](#known-limitations-and-deferred-work)。
+
 ## Model Experience
 
 无：本包是纯文件系统原语，此处没有任何内容会到达模型请求。
@@ -34,3 +42,4 @@ await writeFileAtomic('/home/u/.dsh/settings.yaml', text, { mode: 0o600 })
 
 - **原子但不保证持久**——不对文件或其所在目录做 `fsync`，因此崩溃后可能观察到 rename 被回退。此处的文件型存储在启动时重新读取并重新发布，把持久性留作调用方的策略。
 - **仅支持字符串内容**——在有消费方需要之前，不提供 `Buffer` 或流式形态。
+- **锁按时长而非归属接管**（`TODO(settings-lock-ownership)`）——持有者若慢于陈旧时限，其锁会被等待方打破，而释放又无条件删除该路径，因此慢写入方可能删掉后继者的锁。两个写入方随之重叠，一轮循环的结果可能丢失。陈旧时限远高于本仓库的任何一次写入，因此暴露面是被暂停或被换出的进程；修法是按归属安全地获取与释放。