Spotify: tell cspot when a new track becomes audible

The Mac dropped the device soon after the first track change: we never
called SpircHandler::notifyAudioReachedPlayback(), so cspot's queue did
not advance and the state sent to Spotify stalled.
- The data callback records a boundary (output write position, track
  id) when the track id changes; the session loop (<= 200 ms) calls
  notifyAudioReachedPlayback(id) once playback (ring read position)
  passes it, and notifyAudioEnded() after DEPLETED once the buffer is
  empty. PLAYBACK_START clears boundaries (next data is a new one),
  seek/flush drop pending ones.
- audio_ring exposes its write/read frame counters (wrap-safe compare).
- Verified: 10 min on the Mac without a disconnect, every track change
  notified ("track audible, Spotify notified"), app shows the playing
  track (switches up to ~3-5 s early), 0 underruns.

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

components/spotify/include/spotify.h

@@ -21,9 +21,13 @@ typedef enum {
 // PCM from Spotify: 44.1 kHz stereo s16 interleaved. Return the frames taken (may block).
 typedef size_t (*spotify_pcm_cb_t)(const int16_t *pcm, size_t frames);
 typedef void (*spotify_event_cb_t)(spotify_event_t ev, int value);
+// Output positions in frames (monotonic, wrap at 2^32): written into the output buffer so far, and
+// played out so far. Used to tell Spotify when a new track actually becomes audible.
+typedef uint32_t (*spotify_pos_cb_t)(void);
 
 // Registers the zeroconf endpoints (/spotify_info), starts the session task and applies the config.
-esp_err_t spotify_init(spotify_pcm_cb_t pcm, spotify_event_cb_t event);
+esp_err_t spotify_init(spotify_pcm_cb_t pcm, spotify_event_cb_t event, spotify_pos_cb_t written,
+                       spotify_pos_cb_t played);
 // Re-read the "source" config: enabled (advertised, sessions accepted) when mode is spotify/auto and
 // the client credentials are set; otherwise not advertised and a running session is ended.
 // A new device name ends the session and re-advertises.