Step 7.1: player plumbing (PSRAM ring -> AES67 TX), test tone mode

- main/audio_ring: SPSC ring of interleaved int32 frames in PSRAM
  (4 s at 48 kHz stereo), lock-free with acquire/release counters; the
  TX pull callback never blocks.
- main/player: source selection from source.mode and the pull callback
  for aes67_tx. tone -> the core's PTP-phased 1 kHz tone; off -> silence;
  hls/spotify -> ring with 1 s prefill (silence while buffering is not an
  underrun; running dry is, and prefills again). Status fields
  active_source, source_state, spotify_state, buffer_ms. source config
  applies live.
- New source.mode "tone" (validation, UI dropdown, doc) for commissioning.
- Temporary 440 Hz producer in hls mode (until the HLS player exists).
- Verified: tone 999.7 Hz -18 dBFS; off silent; hls 440.0 Hz with max
  sample step 60816 (ideal sine 60825, i.e. no discontinuities), buffer
  3983 ms, 0 underruns; spotify silent/not implemented.

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

docs/hardware-and-design-notes.md

@@ -30,8 +30,9 @@ source (cspot Spotify Connect | HLS player) -> decode (Vorbis/AAC/MP3) -> SRC 44
 - Firmware needs Spotify events: cspot connect, disconnect, play, pause.
 
 ## Project config group
-- source: {mode: spotify|hls|auto|off, spotify_name, spotify_bitrate, hls_url, autoplay, gain_db, failover_delay_s, failover_on_pause}
+- source: {mode: spotify|hls|auto|tone|off, spotify_name, spotify_bitrate, hls_url, autoplay, gain_db, failover_delay_s, failover_on_pause}
 - Project status fields: active_source, source_state, spotify_state, buffer_ms
+- mode "tone": the core's 1 kHz / -18 dBFS test tone, phase-locked to PTP (commissioning, e.g. Riedel import tests). "off": silence.
 
 ## Player control API (project routes)
 - GET /api/player -> {source, forced, state(playing|paused|stopped|buffering|idle), artist, title, album, position_ms, duration_ms, volume(0-100), can:{pause,next,prev,seek}}