fix: handle macOS gesture button in event tap

[thisislvca]

Summary

Handle the MX gesture/action button on macOS when it comes through the CGEvent tap as button 6.

Changes

• add a macOS _BTN_GESTURE = 6 mapping in core/mouse_hook.py
• route button 6 down/up events into the existing gesture handlers
• block the raw OS event so the button does not fall through unhandled
• add regression coverage for the darwin event-tap path in tests/test_mouse_hook.py

Validation

• python3 -m unittest tests.test_mouse_hook
• python3 -m unittest discover -s tests

Context

This is a follow-up to the MX Master 4 detection work merged in #38, but it is scoped to the macOS mouse-hook path only.