feat(serial): ReaderControl restores ClearBuffer + GetInWaiting (closes #756)

[zackees]

Closes #756. Sub-PR of meta #755.

Summary

PR #750 split handle_serial_ws into reader / writer / inbound tasks for throughput, but the split moved the broadcast receiver rx exclusively into the reader task. As a side effect, ClearBuffer and GetInWaiting (both of which reach into rx) became logged no-ops in the inbound task.

This PR restores both via an internal cross-task control channel.

Design

enum ReaderControl {
    Drain { reply: oneshot::Sender<usize> },
    GetDepth { reply: oneshot::Sender<usize> },
}

• New unbounded mpsc (control_tx, control_rx) created alongside the existing out-mpsc in handle_serial_ws.
• Reader's tokio::select! adds a branch on control_rx.recv(). biased; keeps broadcast forwarding the priority so a burst of inbound control requests cannot starve forwarding.
• Inbound's ClearBuffer handler sends Drain, awaits the oneshot reply with the drop count, logs.
• Inbound's GetInWaiting handler sends GetDepth, awaits the oneshot reply, ships InWaiting { count } through the writer-mpsc (preserves the writer-is-sole-WS-sink invariant from perf(serial): split daemon WS handler into reader/writer/inbound (closes #749) #750).

Race safety: if reader exits between send and recv (session teardown), the oneshot resolves with Err and inbound logs debug / falls back to count=0.

Protocol impact

None. SerialClientMessage::ClearBuffer and SerialClientMessage::GetInWaiting are unchanged on the wire. Only the internal handler implementation moved.

Test plan

• soldr cargo build -p fbuild-daemon — clean
• Integration test on Teensy 4 via FastLED autoresearch (deferred to a follow-up — admin-merging now per the feat(serial): ReaderControl channel to restore ClearBuffer + GetInWaiting #756 plan; hardware validation lives in the FastLED side of the loop).

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between cd25295 and 184af28.

📒 Files selected for processing (1)

• crates/fbuild-daemon/src/handlers/websockets.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/756-reader-control

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