refactor: split monolithic main.c into modular source files

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

src/midi.h

@@ -0,0 +1,8 @@
+#ifndef MIDI_H
+#define MIDI_H
+
+#include <jack/types.h>
+
+void midi_handle_events(void *port_buffer, jack_nframes_t nframes);
+
+#endif