feat: add rack mode, colon commands, and client command parser

client/src/client_cmd.h

@@ -0,0 +1,16 @@
+#ifndef CLIENT_CMD_H
+#define CLIENT_CMD_H
+
+#include <stdbool.h>
+
+/*
+ * Handle a client command (without the leading ':').
+ * Returns 0 on success, -1 on error.
+ * If the command loads/creates a new plugin, *out_id is set to the new ID.
+ * Otherwise *out_id is unchanged.
+ */
+int handle_client_command(const char *input, int *out_id);
+const char* get_stored_from(void);
+const char* get_stored_to(void);
+
+#endif