* Support external monitor brightness
Route brightness through the focused Hyprland monitor so internal panels keep using the kernel backlight while compatible external displays use DDC/CI. Preserve the Apple Display backend and leave brightness unavailable when the focused display cannot be controlled.
Add cached DDC bus and VCP range handling, install ddcutil for new and existing systems, and cover backend selection and brightness conversion with shell tests.
* Harden external brightness caching
---------
Co-authored-by: David Heinemeier Hansson <david@hey.com>
