Why

Initialized app-server RPCs no longer need to bottleneck behind one request processor path. Running them concurrently improves responsiveness, but several request families still mutate shared state or depend on ordered side effects. Those stateful families need an auditable serialization contract so concurrency does not reorder thread, config, auth, command, watcher, MCP, or similar state transitions.

This PR keeps that boundary explicit: stateful work is serialized by the smallest useful key, while intentionally read-only or externally concurrent work remains unkeyed. In particular, thread/list and thread/turns/list explicitly have no serialization because they primarily read append-only rollout storage and should continue to be served concurrently.

What changed

• Adds ClientRequest::serialization_scope() in app-server-protocol and requires every client request definition to declare its serialization behavior.
• Introduces keyed request scopes for thread, thread path, command exec process, fuzzy search session, fs watch, MCP OAuth, and global state buckets such as config, account auth, memory, and device keys.
• Routes initialized app-server RPCs through per-key FIFO serialization while allowing unkeyed initialized requests to run concurrently.
• Cancels in-flight initialized RPC work when the connection disconnects or the app-server exits so spawned request tasks do not outlive their session.
• Adds focused coverage for representative keyed and unkeyed serialization scopes, including explicitly concurrent thread/turns/list behavior.

Validation

• Added protocol tests for representative keyed serialization scopes and intentionally unkeyed request families.
• Added app-server request serialization tests covering per-key FIFO behavior, concurrent unkeyed execution, disconnect shutdown, and config read-after-write ordering.
• Local focused protocol validation after the latest rebase is currently blocked by packageproxy failing to resolve locked rustls-webpki 0.103.13; CI is expected to provide the full validation signal.