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/llm/llm/src/index.ts

@@ -199,6 +199,10 @@ export interface AdapterRegistrationHandle {
    * one synchronous section, so no request can observe a gap. An empty array
    * is legal here (a settings section that emptied holds zero routes while
    * staying registered), unlike an empty initial registration.
+   *
+   * Throws `LlmError` with code `REGISTRATION_DISPOSED` once the registration
+   * has been released: its routes are gone and its disposer has already run,
+   * so anything registered afterwards would have no owner left to release it.
    * @param providers - the complete next route set for this registration.
    */
   replace(providers: string[]): void
@@ -227,10 +231,14 @@ export class LlmService extends Service {
     // The routes this registration currently holds; `replace` rewrites it, and
     // the disposer releases whatever it holds at disposal time.
     const owned = new Set<string>()
+    // The disposer has run: `owned` being empty cannot say so on its own,
+    // because `replace([])` legally leaves a live registration holding none.
+    let released = false
     const dispose = this.ctx.effect(function* (this: LlmService) {
       if (providers.length === 0) throw new LlmError('an adapter must register at least one provider', 'INVALID_ADAPTER')
       this.commitRoutes(owned, this.prepareRoutes(providers, adapter, owned))
       yield () => {
+        released = true
         for (const provider of owned) this.adapters.delete(provider)
         owned.clear()
       }
@@ -239,6 +247,11 @@ export class LlmService extends Service {
     // synchronous fire-and-forget — discard the (always-resolved) promise.
     const handle = (() => void dispose()) as AdapterRegistrationHandle
     handle.replace = (next: string[]): void => {
+      // Registering here would leak: the effect's disposer already ran, so
+      // nothing remains to release whatever this call would put in the map.
+      if (released) {
+        throw new LlmError('a disposed adapter registration cannot replace its routes', 'REGISTRATION_DISPOSED')
+      }
       this.commitRoutes(owned, this.prepareRoutes(next, adapter, owned))
     }
     return handle

packages/llm/llm/tests/service.spec.ts

@@ -1381,4 +1381,32 @@ describe('LlmService', () => {
     disposeAgain()
     expect(ctx.llm.listProviders()).toEqual([])
   })
+
+  it('refuses to replace routes on a registration that was already released', async () => {
+    // The leak this prevents: the effect's disposer has run, so a route added
+    // afterwards would sit in the registry with nothing left to release it.
+    const ctx = new Context()
+    await ctx.plugin(LlmService)
+
+    const handle = ctx.llm.registerAdapter(['m1'], new ScriptedAdapter(SCRIPT))
+    handle()
+    expect(() => { handle.replace(['leaked']) })
+      .toThrow(/disposed adapter registration cannot replace its routes/)
+    expect(ctx.llm.listProviders()).toEqual([])
+  })
+
+  it('still allows an empty route set on a live registration', async () => {
+    // `replace([])` is the settings-section-emptied case: legal, and it must
+    // not be mistaken for disposal by the guard above.
+    const ctx = new Context()
+    await ctx.plugin(LlmService)
+
+    const handle = ctx.llm.registerAdapter(['m1'], new ScriptedAdapter(SCRIPT))
+    handle.replace([])
+    expect(ctx.llm.listProviders()).toEqual([])
+    handle.replace(['m2'])
+    expect(ctx.llm.listProviders()).toEqual([{ id: 'm2', name: 'm2' }])
+    handle()
+    expect(ctx.llm.listProviders()).toEqual([])
+  })
 })