Player: transport commands on /api/player

play/pause/toggle/stop/next/prev. Spotify commands run on the session task
(same thread as cspot's frame handling) and the POST waits until they ran,
so it answers the new state. HLS pause stops fetching and resume rejoins at
the live edge; next/prev on HLS and anything on tone/off return 409.

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

CLAUDE.md

@@ -69,7 +69,7 @@ Repo: https://gitea.apointless.space/bsncubed/aes67-ESP32-P4
     - An old, long-idle session can get out of sync with the app: it sends empty Load frames ("No tracks in frame") instead of Pause/Play, so controls do nothing. Quitting and reopening Spotify on the Mac fixes it.
   - [x] failover (auto mode): no session or (with `failover_on_pause`) paused for `failover_delay_s` -> HLS; Spotify playing -> Spotify within ~1 s; 30 ms fades; HLS suspended while Spotify plays; cspot is held back (not drained) while it isn't Spotify's turn, so it resumes where it paused. Verified: no session -> HLS, play -> Spotify, pause stays (on_pause off), pause -> HLS after 5 s (on_pause on), play -> Spotify at the paused position.
     - Open: `aes67_tx resync` (TX 20-27 ms late) ~11 s after each switch to Spotify (seen 3 times); clicks at switches not yet checked in a recording; HLS start sometimes hits CDN read timeouts.
-  - [ ] /api/player: GET done (source, state, track, position as the app shows it, volume, can). Next: transport POSTs, seek/volume, source override/url.
+  - [ ] /api/player: GET and transport (play/pause/toggle/stop/next/prev) done. Next: seek/volume, source override/url.
 - [ ] 8. Mono sum, gain, polish.
 
 ## Phase 2 (parked)