Propagate client state changes to clients

[whme]

Introduce a per-client state channel that the daemon's pipe server
task subscribes to, replacing the previous PipeServerState mutex.
Each client's connection now multiplexes input records, state-change
notifications, and keep-alives via tokio::select!, sending a
TAG_STATE_CHANGE envelope on every transition.

The wire protocol gains a ClientState enum (currently a single
Active variant) carried as a one-byte payload after TAG_STATE_CHANGE.
The client tracks the last received state in its read/write loop so
future PRs can act on additional variants without further protocol
changes.

This PR is plumbing only — no control-mode bindings or visual
changes are wired up yet. Daemon::set_client_state is added as a
private helper for upcoming work.

GitHub: #179
Co-authored-by: Claude Opus 4.7 noreply@anthropic.com

[Copilot]

Pull request overview

Adds end-to-end plumbing for propagating per-client state changes (daemon → client) over the existing named-pipe protocol, enabling the client main loop to track its authoritative state for future UI/control-mode work (Issue #179).

Changes:

• Introduces ClientState and a new TAG_STATE_CHANGE framed message on the wire.
• Updates daemon pipe server routine to multiplex input records, state updates, and keep-alives via tokio::select!.
• Updates client read/write loop to apply incoming state-change messages and adds unit/integration tests for serialization/parsing and pipe behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/protocol.rs	Adds ClientState, TAG_STATE_CHANGE, and framed-length constant; extends daemon→client message enum.
src/serde/serialization.rs	Adds serialize_client_state and state-change envelope serialization.
src/serde/deserialization.rs	Adds deserialize_client_state and parsing support for TAG_STATE_CHANGE.
src/daemon/mod.rs	Replaces per-client mutex state with watch channel; multiplexes pipe outputs; adds write helper.
src/client/mod.rs	Tracks and updates ClientState in the main loop via StateChange messages.
src/tests/serde/test_mod.rs	Adds round-trip tests for ClientState and framed state-change message.
src/tests/daemon/test_mod.rs	Updates client fixtures for watch sender and adds state-change forwarding test.
src/tests/client/test_mod.rs	Adds test ensuring client loop applies state changes and still forwards input records.

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[Copilot]

write_framed_message retries partial writes by calling try_write(frame) again with the full frame. If try_write ever returns Ok(n) where n < frame.len(), the next iteration will resend the already-written prefix, corrupting the byte stream (and can also lead to an infinite loop if the pipe keeps accepting only a subset). Track an offset and retry with &frame[written..] until fully drained.