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/settings/settings/src/index.ts

@@ -612,6 +612,11 @@ export function installSettingsSection<T>(
     })
     hooks.onChange()
     scope.watch(() => {
+      // A stored change landing while the consumer unloads reaches the watcher
+      // before the registration is released, and `onChange` is exactly as
+      // harmful here as in the disposer above: it re-registers routes against
+      // a fiber whose resources are being let go.
+      if (isUnloading(ctx)) return
       hooks.onChange()
     })
   })

packages/settings/settings/tests/settings.spec.ts

@@ -724,4 +724,36 @@ describe('installSettingsSection', () => {
     await new Promise(resolve => setTimeout(resolve, 20))
     expect(changes).toEqual(['user'])
   })
+
+  it('stays silent for a stored change that lands while the consumer unloads', async () => {
+    // The watcher outlives the start of teardown by the width of the unload,
+    // so a document change arriving in that window reaches it. Notifying then
+    // is exactly as harmful as notifying from the disposer.
+    const { ctx, provider } = await boot({ doc: { 'helper-ns': { theme: 'user' } } })
+    const entry = { theme: 'entry' }
+    let current: () => { theme: string } = () => entry
+    const changes: string[] = []
+    const consumer = ctx.plugin({
+      inject: ['settings'],
+      apply: (child: Context) => {
+        installSettingsSection(child, settingsNamespace('helper-ns'), HelperSchema, entry, {
+          setSource: (source) => {
+            current = source
+          },
+          onChange: () => {
+            changes.push(current().theme)
+          },
+        })
+      },
+    })
+    await consumer
+    await vi.waitFor(() => {
+      expect(changes).toEqual(['user'])
+    })
+
+    const unloading = consumer.dispose()
+    provider.pushExternal({ 'helper-ns': { theme: 'racing' } })
+    await unloading
+    expect(changes).toEqual(['user'])
+  })
 })