feat: add transport.h header with transport state machine and MIDI clock support

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

transport.h

@@ -0,0 +1,92 @@
+#ifndef TRANSPORT_H
+#define TRANSPORT_H
+
+#include <jack/jack.h>
+#include <jack/midiport.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include <stdatomic.h>
+
+#define MIDI_CLOCKS_PER_BEAT 24
+#define BEATS_PER_BAR 4
+#define DEFAULT_BPM 120.0
+
+typedef enum {
+    TRANSPORT_STOPPED,
+    TRANSPORT_PLAYING,
+    TRANSPORT_PAUSED
+} TransportState;
+
+typedef enum {
+    CLOCK_SOURCE_INTERNAL,  // Master clock
+    CLOCK_SOURCE_MIDI       // Slave to external MIDI clock
+} ClockSource;
+
+typedef struct {
+    // State
+    TransportState state;
+    ClockSource clock_source;
+    
+    // Position
+    uint32_t clock_count;
+    uint32_t beat_position;   // 0-3
+    uint32_t bar_position;
+    uint32_t sample_position; // derived from clock at current sample rate
+    
+    // Internal clock generation
+    double bpm;
+    double samples_per_beat;
+    double sample_accumulator;  // For precise BPM timing
+    
+    // Atomic mirrors for frontend reads
+    atomic_int state_atomic;           // TransportState
+    atomic_uint clock_count_atomic;
+    atomic_uint beat_position_atomic;
+    atomic_uint bar_position_atomic;
+    atomic_uint sample_position_atomic;
+    atomic_int clock_source_atomic;    // ClockSource
+    atomic_double bpm_atomic;
+    
+    // JACK ports for MIDI clock output (when master)
+    jack_port_t *midi_clock_out_port;
+    
+    // Reference to engine for sample rate
+    jack_nframes_t sample_rate;
+} Transport;
+
+// Lifecycle
+void transport_init(Transport *transport, jack_nframes_t sample_rate);
+void transport_cleanup(Transport *transport);
+
+// Control
+void transport_play(Transport *transport);
+void transport_pause(Transport *transport);
+void transport_stop(Transport *transport);
+void transport_toggle_play(Transport *transport);
+
+// Clock source
+void transport_set_clock_source(Transport *transport, ClockSource source);
+ClockSource transport_get_clock_source(Transport *transport);
+
+// BPM (only relevant for internal clock)
+void transport_set_bpm(Transport *transport, double bpm);
+double transport_get_bpm(Transport *transport);
+
+// Process function called from audio thread
+// Returns number of MIDI clock ticks generated (for master mode)
+int transport_process(Transport *transport, jack_nframes_t nframes, 
+                      void *midi_clock_in_buf, void *midi_clock_out_buf);
+
+// Reset position
+void transport_reset(Transport *transport);
+
+// Get next quantization boundary (in frames from current position)
+jack_nframes_t transport_get_next_quantize_frame(Transport *transport, 
+                                                  jack_nframes_t current_frame,
+                                                  QuantizeMode mode);
+
+// Utility
+const char* transport_state_to_string(TransportState state);
+const char* clock_source_to_string(ClockSource source);
+
+#endif // TRANSPORT_H