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/host/apiproxy/tests/fetch-carrier.spec.ts

@@ -155,6 +155,36 @@ function fakeApi(overrides: Partial<{ muxFrames: MuxFrame[]; hostFrames: HostFra
         return { rpcId: request.rpcId, result: { ok: false, error: { code: 'internal', message: 'stub', details: {} } } }
       },
     },
+    settings: {
+      async describe(request) {
+        return { rpcId: request.rpcId, result: { ok: true, value: { writable: true, namespaces: [] } } }
+      },
+      async update(request) {
+        return { rpcId: request.rpcId, result: { ok: false, error: { code: 'settings-rejected', message: 'stub', details: { ns: request.payload.ns } } } }
+      },
+      async replace(request) {
+        return { rpcId: request.rpcId, result: { ok: false, error: { code: 'settings-rejected', message: 'stub', details: { ns: request.payload.ns } } } }
+      },
+    },
+    credentials: {
+      async describe(request) {
+        return { rpcId: request.rpcId, result: { ok: true, value: { credentials: {} } } }
+      },
+      async set(request) {
+        return { rpcId: request.rpcId, result: { ok: true, value: {} } }
+      },
+      async unset(request) {
+        return { rpcId: request.rpcId, result: { ok: true, value: {} } }
+      },
+    },
+    llm: {
+      async providers(request) {
+        return { rpcId: request.rpcId, result: { ok: true, value: { providers: [] } } }
+      },
+      async models(request) {
+        return { rpcId: request.rpcId, result: { ok: true, value: { groups: [], failures: [] } } }
+      },
+    },
     events: {
       mux: (_request, signal) => stream(muxFrames, signal),
       host: (_request, signal) => stream(hostFrames, signal),