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>

main/main.c

@@ -10,6 +10,7 @@
 #include "esp_app_desc.h"
 #include "esp_chip_info.h"
 #include "esp_log.h"
+#include "player.h"
 #include "project_cfg.h"
 
 static const char *TAG = "main";
@@ -39,6 +40,7 @@ void app_main(void)
     project_cfg_register();
     ESP_ERROR_CHECK(aes67_ota_init());
     ESP_ERROR_CHECK(aes67_sdp_sap_init());
+    ESP_ERROR_CHECK(player_init());
     ESP_ERROR_CHECK(aes67_tx_start());
     ESP_ERROR_CHECK(aes67_web_start());
 }