Target built-in brightness controls to the active display

[adarsh-crypto]

Summary

• route built-in brightness reads and writes through an explicit target display ID instead of always falling back to the main display
• prefer the screen under the pointer, then the selected/preferred Boring Notch display, before falling back to the helper's existing built-in display resolution
• keep the brightness OSD attached to the same screen that receives the brightness change

Changes

• / XPC protocols: add targeted screen brightness read/write methods while preserving the existing built-in fallback path
• : pass optional display IDs through the helper boundary
• : resolve the active display for brightness key presses and slider changes, then refresh/show the OSD for that same display
• : expose a reusable display ID helper

Notes

• this change only improves the built-in macOS brightness control path
• it does not add DDC/CI support for external monitors that require hardware-specific brightness control

Test plan

• Build in Release configuration on macOS
• With multiple displays connected, move the pointer to the built-in display and press brightness keys
• Change the preferred Boring Notch display and verify the brightness OSD appears on the controlled screen
• Confirm unsupported external monitors still fall back cleanly without regressing built-in display brightness