feat: add save/load thread and WAV file I/O for clip persistence

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

engine.h

@@ -6,6 +6,7 @@
 #include <stdint.h>
 #include <stdbool.h>
 #include <stdatomic.h>
+#include <pthread.h>
 #include "transport.h"
 
 #define MAX_SCENES 8
@@ -114,6 +115,25 @@ typedef struct {
     atomic_uint read_index;
 } CommandQueue;
 
+// Save/Load request types
+typedef enum {
+    REQ_SAVE_CLIP,
+    REQ_LOAD_CLIP
+} SaveLoadType;
+
+typedef struct {
+    SaveLoadType type;
+    int clip_index;
+    char filename[256];
+} SaveLoadRequest;
+
+// Lock-free queue for save/load requests (audio thread -> save/load thread)
+typedef struct {
+    SaveLoadRequest buffer[MAX_QUEUED_COMMANDS];
+    atomic_uint write_index;
+    atomic_uint read_index;
+} SaveLoadQueue;
+
 // Queued trigger for quantization
 typedef struct QueuedTrigger {
     int clip_index;
@@ -154,6 +174,11 @@ typedef struct {
     
     // Undo/Redo
     UndoHistory undo_history;
+    
+    // Save/Load queue and thread
+    SaveLoadQueue save_load_queue;
+    pthread_t save_load_thread;
+    volatile bool save_load_running;
 } Engine;
 
 // Engine lifecycle
@@ -189,6 +214,16 @@ void engine_process_commands(Engine *engine);
 // Initialize command queue (exposed for testing)
 void command_queue_init(CommandQueue *q);
 
+// Save/Load queue management
+void save_load_queue_init(SaveLoadQueue *q);
+int save_load_queue_push(SaveLoadQueue *q, SaveLoadType type, int clip_index, const char *filename);
+int save_load_queue_pop(SaveLoadQueue *q, SaveLoadRequest *req);
+
+// Save/Load thread
+void* save_load_thread_func(void *arg);
+int engine_start_save_load_thread(Engine *engine);
+void engine_stop_save_load_thread(Engine *engine);
+
 // Utility
 const char* clip_state_to_string(ClipState state);
 uint8_t clip_state_to_velocity(ClipState state);