docs(api-reference): document the "stopping" playback state (firmware F-065)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/pages/engineering/developer/ApiReferencePage.jsx

@@ -19,7 +19,7 @@ const TRANSPORTS_V2 = [
   { transport: 'MQTT', address: 'vesper/{device_id}/control/ack', direction: 'Outbound, QoS 1', switch: 'command', notes: 'Strictly replies to commands (incl. pong) — nothing unsolicited is ever published here. Echoes req_id when the request sent one.' },
   { transport: 'MQTT', address: 'vesper/{device_id}/control/reports', direction: 'Outbound, QoS 1', switch: 'reports', notes: 'Critical, unsolicited, time-sensitive board-initiated events (e.g. bell_overload). Not retained.' },
   { transport: 'MQTT', address: 'vesper/{device_id}/status/heartbeat', direction: 'Outbound, retained, QoS 1', switch: 'heartbeat', notes: 'Every 30 s. Also carries LWT — broker publishes {"state":"offline","ok":false} on an unclean disconnect.' },
-  { transport: 'MQTT', address: 'vesper/{device_id}/status/playback', direction: 'Outbound, retained, QoS 1', switch: 'playback', notes: 'Playback state transitions (playing/paused/idle) with pid, speed, duration, started_at, source, ts. Published on transition, not polled. Source of truth for what the board is doing.' },
+  { transport: 'MQTT', address: 'vesper/{device_id}/status/playback', direction: 'Outbound, retained, QoS 1', switch: 'playback', notes: 'Playback state transitions (playing/paused/stopping/idle) with pid, speed, duration, started_at, source, ts. Published on transition, not polled. Source of truth for what the board is doing.' },
   { transport: 'MQTT', address: 'vesper/{device_id}/system/alerts', direction: 'Outbound, retained until cleared, QoS 1', switch: 'alerts', notes: 'Subsystem state changes (WARNING / CRITICAL / FAILED / CLEARED). Published on transition only.' },
   { transport: 'MQTT', address: 'vesper/{device_id}/system/info', direction: 'Outbound, retained, QoS 1', switch: 'info', notes: 'Discrete system events, e.g. boot_report on every boot.' },
   { transport: 'MQTT', address: 'vesper/{device_id}/system/logs', direction: 'Outbound, QoS 0', switch: 'logs', notes: 'Full log stream, mirrors serial output. Also gated by the log.set_mqtt level, independently of this switch.' },
@@ -1858,7 +1858,7 @@ function TransportsV2() {
           </p>
           <div style={{ display: 'grid', gridTemplateColumns: 'repeat(auto-fit, minmax(160px, 1fr))', gap: 'var(--space-3)' }}>
             {[
-              { field: 'action', type: 'string', notes: '"playing" | "paused" | "idle". Published on transition, not polled.' },
+              { field: 'action', type: 'string', notes: '"playing" | "paused" | "stopping" | "idle". paused = between interval segments (still active). stopping = duration reached, bells finishing the current melody loop (up to one loop long), then idle. playback.stop always goes straight to idle. Published on transition, not polled.' },
               { field: 'time_elapsed', type: 'int', notes: 'Seconds since playback started, as of ts (0 for "idle").' },
               { field: 'projected_run_time', type: 'int', notes: 'Projected total run time in ms (0 if not applicable).' },
               { field: 'pid', type: 'string', notes: 'Melody being played. "" when idle.' },