feat: add input priming to verify data flow before streaming

- Read one frame from each non-precoded input to validate audio sources are working
- Store primed frames for drift compensation crossfading when enabled
- Add logging to track successful prime reads and diagnose potential input issues

src/auracast/multicast.py

@@ -703,6 +703,23 @@ class Streamer():
         last_stats_log = time.perf_counter()
         stats_interval = 5.0  # Log stats every 5 seconds
         frame_count = 0
+        # Prime inputs: read one frame from each non-precoded input to verify data flow
+        try:
+            for j, _big in enumerate(bigs.values()):
+                if not _big.get('precoded'):
+                    gen = _big.get('frames_gen')
+                    if gen is None:
+                        gen = _big['audio_input'].frames(_big['lc3_frame_samples'])
+                        _big['frames_gen'] = gen
+                    test_frame = await anext(gen, None)
+                    logging.info(
+                        f"Prime read BIG{j}: bytes={0 if test_frame is None else len(test_frame)} samples={_big['lc3_frame_samples']}"
+                    )
+                    # Store for crossfade if needed
+                    if enable_drift_compensation and test_frame is not None:
+                        _big['prev_pcm_frame'] = test_frame
+        except Exception as e:
+            logging.error(f"Prime read failed: {e}", exc_info=True)
         
         # One streamer fits all
         while self.is_streaming: