[client] audio: minimize LGMP playback latency

Scope the legacy latency offset to classic SPICE and use the LGMP
poll bound as its fixed arrival reserve. Keep learned jitter additive.

Select provider clock feedback only when the current remote endpoint
advertises it, avoiding a failed startup attempt on broker v1.

client/include/interface/audio.h

@@ -131,10 +131,24 @@ typedef struct LG_AudioEventOps
 }
 LG_AudioEventOps;
 
+typedef enum LG_AudioPlaybackFlag
+{
+  /* Apply audio:latencyOffset when local playback rate control is active.
+   * This preserves the legacy safety margin required by classic SPICE. */
+  LG_AUDIO_PLAYBACK_LEGACY_LATENCY_OFFSET = 1U << 0,
+}
+LG_AudioPlaybackFlag;
+
 typedef struct LG_AudioOps
 {
   const char * name;
 
+  /* A zero arrival reserve selects the client's default. Providers with a
+   * tighter, measured delivery bound may request a smaller value. Runtime
+   * delivery jitter is learned and added independently. */
+  uint32_t playbackFlags;
+  uint32_t playbackArrivalReserveUs;
+
   /* Registration must synchronously report the current status after releasing
    * any backend locks. Passing NULL unregisters the listener. Providers with
    * no status listener are assumed to remain available while their transport
@@ -169,6 +183,11 @@ typedef struct LG_AudioOps
    * restart playback with local rate control. */
   bool (*clockFeedback)(void * opaque, uint32_t generation,
       const LG_AudioClock * playbackClock, double targetRate);
+
+  /* Optional nonblocking capability check used before selecting provider
+   * rate control for a new stream. If omitted, clockFeedback is assumed to
+   * be available. */
+  bool (*clockFeedbackAvailable)(void * opaque);
 }
 LG_AudioOps;