tid_key_probe

Measures how much of the sun the squall is actually eating, for part 9 of Raising Preseed ("TIDES", the Farjan).

It marches eight rays from the ferry toward the sun through tid_cloud_vol — the very density volume tid_cloud_fill bakes and tid_cloud_march draws — accumulating optical depth at the same density gain the march uses, and writes the mean transmittance to outT[0] and the darkest single ray to outT[1]. preseed/tides/light.module reads those back and gates the directional key with them, which is what makes the shadow of a particular cloud crossing her darken the ship instead of a schedule doing it.

The rays are spread across the sun's bearing at two heights so that a cloud edge sweeps the key as a ramp the length of the ship rather than popping. outT[1] is published alongside the mean so a caller can tell a shadow edge (mean high, minimum low) from evenly overcast sky (both low).

The uniform layout is documented at the head of shader.wgsl and is packed by light.module; both sides must be edited together.