refactor: split monolithic main.c into modular source files

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

src/looper.h

@@ -0,0 +1,18 @@
+#ifndef LOOPER_H
+#define LOOPER_H
+
+#include <jack/jack.h>
+
+/* Initialisation – must be called after setting process callback */
+int looper_init(jack_client_t *client);
+
+/* Process callback – to be called by JACK */
+int process_callback(jack_nframes_t nframes, void *arg);
+
+/* Shutdown callback */
+void jack_shutdown_cb(void *arg);
+
+/* Main‑loop command processing (add/remove channels) */
+void looper_process_commands(jack_client_t *client);
+
+#endif