fix: prevent hang in scene add/remove test and fix unsafe scene copy

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

src/channel.c

@@ -91,9 +91,18 @@ void channel_remove_scene(jack_client_t *client, int idx) {
     if (cur[idx].scene_count <= 1)
         return;
     int cs = cur[idx].current_scene;
-    /* shift remaining scenes down */
+    /* shift remaining scenes down (safe copy of fields) */
     for (int i = cs; i < cur[idx].scene_count - 1; i++) {
-        cur[idx].scenes[i] = cur[idx].scenes[i + 1];
+        cur[idx].scenes[i].loop_count = cur[idx].scenes[i+1].loop_count;
+        cur[idx].scenes[i].record_pos = cur[idx].scenes[i+1].record_pos;
+        cur[idx].scenes[i].playback_pos = cur[idx].scenes[i+1].playback_pos;
+        atomic_store(&cur[idx].scenes[i].state,
+                     atomic_load(&cur[idx].scenes[i+1].state));
+        cur[idx].scenes[i].prev_state = cur[idx].scenes[i+1].prev_state;
+        /* copy loop data */
+        memcpy(cur[idx].scenes[i].loop.audio_buffer,
+               cur[idx].scenes[i+1].loop.audio_buffer,
+               LOOP_BUF_SIZE * sizeof(float));
     }
     cur[idx].scene_count--;
     if (cur[idx].current_scene >= cur[idx].scene_count)