feat(server): add opt-in latest-state scheduling and stop barriers

[xmsama]

Summary

Implements the output-scheduling scope approved in #954.

• Add an opt-in latest-state scheduler with one hardware-write batch in flight and coalescing of superseded pending commands.
• Preserve explicit stops and zero-valued intensity commands as urgent barriers; subsequent motion cannot overwrite an unsent stop. Position zero remains a valid endpoint.
• Bound individual writes to two seconds. On write failure, timeout, or hardware disconnect, discard pending motion rather than retrying it on the same connection.
• Force explicit stop generation only for opted-in protocols. Preserve the existing scheduler, keepalive behavior, and stop deduplication for other protocols.

This PR adds the infrastructure only. The companion Yiciyuan PR enables it for FJB-02/FJB-03. It introduces no zero-value hold, Bluetooth connection retries, or client/UI changes.

Validation

Windows, Rust 1.96.0:

• cargo test -p buttplug_server --lib: 42 passed.
• cargo test -p buttplug_tests --test test_device_protocols: 861 passed.
• Scheduler tests cover slow-write coalescing, single-flight ordering, stop barriers, pacing bypass, write failure, write timeout, and disconnect during a blocked write.

These counts are suite totals. Tests use simulated hardware. A completed OS write is not a device execution acknowledgement; a failed or disconnected BLE link cannot guarantee physical motor stop.