Investigate per-monitor control

[michaelmawhinney]

Investigate whether RedLight can support per-monitor control.

Potential behavior:

• apply red mode to all monitors
• apply red mode only to selected monitor(s)
• show monitor/backend state in diagnostics

This may be limited by the Magnification API and should be investigated before committing to implementation.

[kneild]

This would be a very useful feature for a planetarium setup where one 'monitor' is possibly the dome screen/projector and would not have the red filter applied.

[codex20260221]

Investigation suggests per-monitor control is feasible, but not through the current MagSetFullscreenColorEffect path, which applies one color transform to the entire desktop.

A promising approach is to keep the current fullscreen Magnification API backend for “all monitors,” and use one 1:1 WC_MAGNIFIER control per selected monitor when only a subset should be filtered. Each magnifier can apply the existing Strict Red / Luma Red color matrix independently, while unselected monitors remain untouched.

Recommended next step: build a small isolated probe under experiments/ that applies the red transform to one monitor only and validates:

• monitor isolation
• click/keyboard passthrough
• cursor behavior
• mixed DPI / monitor coordinates
• taskbar/menus/video/hardware acceleration
• sleep/wake and hotplug
• performance

If that probe is reliable, the feature looks practical without replacing RedLight’s current display architecture.