lut post-process shader

Colour grading through a lookup table. pp.add(name, "lut", { texture = ... }) registers it; @builtin::modules.lut builds, imports and installs the tables it reads.

lut_texture is a LUT strip: an N-entry colour cube laid out as one row of N square tiles, so the image measures N*N wide by N tall. Tile b holds the plane of constant blue index, red runs across a tile and green runs down it. N comes from the texture's own height, so this one shader serves a 16-, 32- or 64-entry cube.

The eight lattice entries around a colour are fetched with textureLoad at exact texel coordinates and interpolated in the shader. That makes the result independent of the texture's filter and wrap mode, and each blue plane is read from its own tile.

A texture whose shape is not a strip — including the 1×1 fallback bound when the effect has no table — returns the frame untouched, so adding the effect before its table exists changes nothing.

Property	Meaning
amount	0 = the ungraded frame, 1 = the table's answer.
lut_texture	The strip.