feat(apiproxy): workspace.archiveSession RPC and archived-sessions frame

workspace.archiveSession answers the full updated archive set;
workspace.list carries the set as the reconnect baseline; the host
stream pushes host/archived-sessions-changed full snapshots from the
domain/changed global-put branch (same posture as workspace-changed).
Unknown sessions map to the existing session-not-found code.

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

@@ -122,7 +122,7 @@ function fakeApi(overrides: Partial<{ muxFrames: MuxFrame[]; hostFrames: HostFra
     },
     workspace: {
       async list(request) {
-        return { rpcId: request.rpcId, result: { ok: true, value: { items: [] } } }
+        return { rpcId: request.rpcId, result: { ok: true, value: { items: [], archivedSessionIds: [] } } }
       },
       async create(request) {
         return {
@@ -145,6 +145,9 @@ function fakeApi(overrides: Partial<{ muxFrames: MuxFrame[]; hostFrames: HostFra
           result: { ok: true, value: { workspace: { workspaceId: 'w1' as never, path: '/w', title: 'w', sessionIds: [], createdAt: 't', updatedAt: 't' } } },
         }
       },
+      async archiveSession(request) {
+        return { rpcId: request.rpcId, result: { ok: true, value: { archivedSessionIds: [request.payload.sessionId] } } }
+      },
     },
     commands: {
       async list(request) {