Step 7.5b3b: Spotify audio on AES67

- spotify_init(pcm_cb, event_cb): cspot's data callback hands 44.1 kHz
  stereo s16 PCM to the player and returns what was taken; the player's
  blocking ring write paces decoding to playback speed.
- Events: FLUSH/SEEK/PLAYBACK_START empty the ring (skip/seek sound at
  once); PLAY_PAUSE pauses output (silence, buffer kept, no underrun);
  VOLUME logged (applied in b4).
- player: Spotify via its own audio_out converter (44.1 -> 48 kHz);
  source_state playing / paused / buffering.
- Verified from a Mac: music on the AES67 stream (10 s: 0 gaps, no
  silent packets, RMS -9.6 dBFS, peak 0.0 dBFS), buffer 4.0 s full,
  0 underruns, pause/play/skip/seek events.
  Seen: first 3 connects "Can't connect to spotify servers", 4th worked.

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

components/spotify/include/spotify.h

@@ -1,15 +1,29 @@
 // Spotify Connect (cspot). Project code; GPL-3.0 through cspot.
 #pragma once
 
+#include <stddef.h>
+#include <stdint.h>
+
 #include "esp_err.h"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+typedef enum {
+    SPOTIFY_EV_PLAY,        // playback (re)started or resumed
+    SPOTIFY_EV_PAUSE,
+    SPOTIFY_EV_FLUSH,       // skip, seek, new track: drop buffered audio
+    SPOTIFY_EV_VOLUME,      // value: 0..65535
+} spotify_event_t;
+
+// 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);
+
 // Registers the zeroconf endpoints (/spotify_info) and, when source.mode is spotify/auto and the
 // client credentials are set, advertises the device and starts the session task.
-esp_err_t spotify_init(void);
+esp_err_t spotify_init(spotify_pcm_cb_t pcm, spotify_event_cb_t event);
 // Current state for status.spotify_state.
 const char *spotify_state(void);