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>

main/audio_ring.c

@@ -70,6 +70,16 @@ size_t audio_ring_read(int32_t *frames, size_t n)
     return n;
 }
 
+uint32_t audio_ring_written(void)
+{
+    return __atomic_load_n(&s_wr, __ATOMIC_ACQUIRE);
+}
+
+uint32_t audio_ring_read_pos(void)
+{
+    return __atomic_load_n(&s_rd, __ATOMIC_ACQUIRE);
+}
+
 void audio_ring_flush(void)
 {
     __atomic_store_n(&s_rd, __atomic_load_n(&s_wr, __ATOMIC_ACQUIRE), __ATOMIC_RELEASE);