feat: add JACK audio looper with clip state machine and tests

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

engine.h

@@ -0,0 +1,56 @@
+#ifndef ENGINE_H
+#define ENGINE_H
+
+#include <jack/jack.h>
+#include <jack/midiport.h>
+#include <stdint.h>
+#include <stdbool.h>
+
+#define MAX_CLIPS 128
+#define MAX_BUFFER_SIZE 441000  // 10 seconds at 44.1kHz
+
+typedef enum {
+    CLIP_EMPTY,
+    CLIP_RECORDING,
+    CLIP_LOOPING,
+    CLIP_STOPPED
+} ClipState;
+
+typedef struct {
+    ClipState state;
+    float *buffer;
+    size_t buffer_size;
+    size_t write_position;
+    size_t read_position;
+    bool is_playing;
+} Clip;
+
+typedef struct {
+    jack_client_t *client;
+    jack_port_t *audio_in_port;
+    jack_port_t *audio_out_port;
+    jack_port_t *midi_in_port;
+    jack_port_t *midi_out_port;
+    
+    Clip clips[MAX_CLIPS];
+    int control_channel;
+    jack_nframes_t sample_rate;
+    
+    bool running;
+} Engine;
+
+// Engine lifecycle
+int engine_init(Engine *engine, const char *client_name);
+void engine_cleanup(Engine *engine);
+int engine_start(Engine *engine);
+void engine_stop(Engine *engine);
+
+// Clip management
+void engine_trigger_clip(Engine *engine, int clip_index);
+void engine_reset_clip(Engine *engine, int clip_index);
+
+// Utility
+const char* clip_state_to_string(ClipState state);
+uint8_t clip_state_to_velocity(ClipState state);
+
+#endif // ENGINE_H