feat: implement lock-free command queue and atomic state for thread safety

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

engine.h

@@ -5,6 +5,7 @@
 #include <jack/midiport.h>
 #include <stdint.h>
 #include <stdbool.h>
+#include <stdatomic.h>
 
 #define MAX_SCENES 8
 #define MAX_CHANNELS 8
@@ -46,6 +47,31 @@ typedef struct {
     bool is_playing;
 } Clip;
 
+// Maximum number of queued commands from frontend to audio thread
+#define MAX_QUEUED_COMMANDS 64
+
+typedef enum {
+    CMD_TRIGGER_CLIP,
+    CMD_TRIGGER_SCENE,
+    CMD_RESET_CLIP,
+    CMD_SET_QUANTIZE_MODE,
+    CMD_SET_QUANTIZE_THRESHOLD,
+    CMD_RESET_TRANSPORT
+} CommandType;
+
+typedef struct {
+    CommandType type;
+    int index;           // clip_index, scene_index, or mode value
+    jack_nframes_t value; // threshold value or other numeric param
+} Command;
+
+// Lock-free single-producer single-consumer ring buffer
+typedef struct {
+    Command buffer[MAX_QUEUED_COMMANDS];
+    atomic_uint write_index;
+    atomic_uint read_index;
+} CommandQueue;
+
 // Queued trigger for quantization
 typedef struct QueuedTrigger {
     int clip_index;
@@ -76,6 +102,18 @@ typedef struct {
     jack_nframes_t quantize_threshold;  // in samples (lookahead)
     QueuedTrigger *queued_triggers;
     
+    // Thread-safe command queue for frontend -> audio thread communication
+    CommandQueue command_queue;
+    
+    // Atomic flags for simple state that frontend reads
+    atomic_int transport_rolling;      // bool
+    atomic_uint transport_clock_count;
+    atomic_uint transport_beat_position;
+    atomic_uint transport_bar_position;
+    atomic_uint transport_sample_position;
+    atomic_int quantize_mode_atomic;   // QuantizeMode
+    atomic_uint quantize_threshold_atomic;
+    
     bool running;
 } Engine;
 
@@ -98,6 +136,12 @@ void engine_reset_transport(Engine *engine);
 // Queue management (exposed for testing)
 void queue_trigger(Engine *engine, int clip_index, bool is_scene, jack_nframes_t time);
 
+// Thread-safe command submission (called from frontend threads)
+int engine_submit_command(Engine *engine, CommandType type, int index, jack_nframes_t value);
+
+// Process pending commands (called from audio thread)
+void engine_process_commands(Engine *engine);
+
 // Utility
 const char* clip_state_to_string(ClipState state);
 uint8_t clip_state_to_velocity(ClipState state);