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()
     })
   })