feat: add a daemon command to stop the current utterance

[StuBehan]

Summary

stop shuts the whole daemon down (forcing a ~2s model reload on the next call), so there was no way to interrupt a long read without paying that cost — and on-demand callers (speaklast, stack-nudge) had no "shush" at all. Add a cancel command that stops the current utterance and drops the queue while leaving the daemon running.

Changes

• {"command": "cancel"} daemon request → _DaemonState.cancel() drains the queue and aborts the in-flight playback via the engine's existing Stackvox.stop().
• daemon.cancel() client helper.
• stackvox cancel CLI subcommand — distinct from stop, which shuts the daemon down.
• Daemons that predate the command reply with an error, so callers degrade safely (a no-op).

No change to the model, playback engine, or existing commands.

Testing

• pytest — adds a protocol test (cancel replies ok, aborts playback via the mocked engine, and the daemon stays up — a follow-up ping still returns ok) and a CLI handler test; full suite green (194).
• ruff check . / ruff format --check . clean.

Related issues

Consumers that send this to interrupt a read without a daemon restart: stackone-speaklast's speaklast --stop, and a stack-nudge "speak selection" hotkey's stop. Both degrade to a no-op against daemons older than this.