//! Projection-decal compute pass.
//!
//! Runs once per frame after the opaque material pipelines have
//! settled `opaque_tex`. Per pixel: reconstruct world position from
//! depth, iterate every active decal, and alpha-blend the decal's
//! sampled texel into the existing `opaque_tex` value. Skipped
//! entirely on frames with no active decals.