rc_gather

Fills the cache, one slot per thread and a fraction of the table per frame.

Runs over cache slots rather than pixels, which is what the cache buys: a thousand pixels looking at one wall share one gather, and that gather is not repeated next frame. Each frame visits the slots whose index falls on this frame's residue, so the table turns over every stride frames.

What a slot gathers is a form-factor sum over the surrounding pixels, averaged into what the slot already holds. Also releases slots nothing has asked for in maxAge frames.