feat(apiproxy): settings/credentials/llm wire domains, frames, and write guard

Eight compiler-locked methods: settings.describe/update/replace serve
redacted layered namespace views (secrets structurally absent from every
layer, write-only in the update direction) and fold seam refusals into
settings-rejected; credentials.describe/set/unset expose value-free views
with credential-rejected on shadowed writes; llm.providers merges the
configurable directory with live routes and llm.models claims the
host-scoped catalog reservation through the buildModelCatalog extraction
session.models now shares. Three HostFrame invalidations bridge the seam
events (host/settings-changed, host/credentials-changed,
host/models-changed), and the connection route generalizes the native-
dialog check into a privileged-method set covering all four writes. The
fixture and both fake clients grow the same face.

packages/client/runtime/tests/fake-api.ts

@@ -162,6 +162,23 @@ export class FakeApiClient implements IApiClient {
     clear: payload => this.record('goal.clear', payload, Promise.resolve(ok({ cleared: true as const }))),
   }
 
+  readonly settings: IApiClient['settings'] = {
+    describe: payload => this.record('settings.describe', payload, Promise.resolve(ok({ writable: true, namespaces: [] }))),
+    update: payload => this.record('settings.update', payload, Promise.resolve(ok({ ns: 'fake', schema: {}, value: {}, applies: 'live' as const, secrets: [] }))),
+    replace: payload => this.record('settings.replace', payload, Promise.resolve(ok({ ns: 'fake', schema: {}, value: {}, applies: 'live' as const, secrets: [] }))),
+  }
+
+  readonly credentials: IApiClient['credentials'] = {
+    describe: payload => this.record('credentials.describe', payload, Promise.resolve(ok({ credentials: {} }))),
+    set: payload => this.record('credentials.set', payload, Promise.resolve(ok({}))),
+    unset: payload => this.record('credentials.unset', payload, Promise.resolve(ok({}))),
+  }
+
+  readonly llm: IApiClient['llm'] = {
+    providers: payload => this.record('llm.providers', payload, Promise.resolve(ok({ providers: [] }))),
+    models: payload => this.record('llm.models', payload, Promise.resolve(ok({ groups: [], failures: [] }))),
+  }
+
   /** When true, streams never fire onOpen (misbehaving-carrier material for the handshake timeout guard). */
   suppressStreamOpen = false