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.

docs/core-data-structures/core.md

@@ -201,6 +201,10 @@ 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