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/user_config.h

@@ -8,11 +8,13 @@ char *get_user_config_path(char *buffer, size_t buflen);
 // Write ~/.vbx.json in new format
 int write_user_config(const char *path, const char *keyboard_sound,
                       const char *mouse_sound, int keyboard_volume,
-                      int mouse_volume, int keyboard_enabled, int mouse_enabled);
+                      int mouse_volume, int keyboard_enabled, int mouse_enabled,
+                      int system_volume_following);
 
 // Read ~/.vbx.json supporting new and legacy formats
 int read_user_config(const char *path, char **out_keyboard_sound,
                      char **out_mouse_sound, int *out_keyboard_volume,
-                     int *out_mouse_volume, int *out_keyboard_enabled, int *out_mouse_enabled);
+                     int *out_mouse_volume, int *out_keyboard_enabled, int *out_mouse_enabled,
+                     int *out_system_volume_following);
 
 #endif // VBX_CONFIG_IO_H