feat: add WAV load/save and ring buffer implementation

Co-authored-by: aider (deepseek/deepseek-reasoner) <aider@aider.chat>

src/channel.c

@@ -28,6 +28,7 @@ void channel_add(jack_client_t *client, int idx) {
   channels[idx].loop_count = 0;
   channels[idx].record_pos = 0;
   channels[idx].playback_pos = 0;
+  channels[idx].save_ring = NULL;
 
   next_channel_id++;
   channel_count++;