feat(brightness): add explicit backlight device override per monitor

[ppenguin]

Disclaimer

Entirely AI generated PR, currently lack the bandwidth to go deep, but it seems to work (see Tested).

Summary

• Adds backlight_device config option to [brightness.monitor.<name>] sections, allowing users to pin a specific sysfs backlight device (e.g. intel_backlight) to a monitor
• When set, the brightness service skips all other backlight devices for that connector, preventing ambiguous multi-device selection
• Adds a new IPC command brightness-list-backlight-devices to enumerate available sysfs device names, making it easy to discover the right value to configure
• Documents the option in example.toml with a usage hint pointing to the discovery command

Tested

Tested on NixOS apple-silicon (asahi base) on MBP M1, which doesn't work without this fix since it targets the touchbar for brightness.

Uses this (nix) config:

      programs = {
        noctalia.settings.brightness.monitor = {
          "eDP-1" = {
            backlight_device = "/sys/class/backlight/apple-panel-bl";
          };
          "touchbar" = {
            # dummy output, not actually reported by niri
            backlight_device = "/sys/class/backlight/228600000.dsi.0"; # touchbar
          };
     ....

🤖 Generated with Claude Code