feat(include): add system volume following support

Implement configurable system volume following and replace JSON IPC with binary struct.

- New Feature: Introduce `system_volume_following` CLI and runtime option.
- Enhancement: Extend CLI options, reload, and user‑config APIs to propagate the new flag.
- Enhancement: Add `VbxEvent` binary IPC struct and logging macros for better performance.
- Enhancement: Add helper to persist system volume following state.
- Bug Fix: Remove stale `parse_keyboard_event` prototype.

include/app/process.h

@@ -17,7 +17,8 @@ void stop_children(void);
 int start_children(const char *sound_dir, const char *config_path, int volume,
                    int verbose, int mute, const char *mouse_sound_dir,
                    const char *mouse_config_path, int mouse_volume,
-                   int keyboard_mute, int mouse_mute, int keyboard_enabled, int mouse_enabled);
+                   int keyboard_mute, int mouse_mute, int keyboard_enabled,
+                   int mouse_enabled, int system_volume_following);
 void handle_sighup(int sig);
 
 #endif // VBX_PROCESS_H