fx_probe_reduce

Reduces the composited frame to a grid of mean-colour tiles, one textureStore per tile, reading @scene.color and writing an rgba8 storage target.

The grid is what makes a reading spatial. A single number over a whole frame cannot tell an effect that lit one corner from one that lit the whole screen, so it cannot answer whether a size parameter changed a size. A tile grid keeps the position of every reading, and the area of the region above a threshold follows from it.

Driven by fx_frame_probe.renderFeature, which owns the target and enqueues this pass after lighting on the ordinary viewport.