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>

web/index.html

@@ -64,6 +64,7 @@ e.g. curl -X POST http://p4-aes67.local/api/player/next
   <option value="spotify">Spotify Connect</option>
   <option value="hls">HLS / M3U8 URL</option>
   <option value="auto">Spotify, fail over to HLS</option>
+  <option value="tone">Test tone (1 kHz, -18 dBFS)</option>
   <option value="off">Off (silence)</option></select></label>
 <label><span>Failover delay (s)</span><input name="source.failover_delay_s" type="number" min="0" max="3600"></label>
 <label><span>Also fail over when paused</span><input name="source.failover_on_pause" type="checkbox"></label>