Material extension for holdout/occluder rendering.
Writes depth to the depth buffer while outputting fully transparent color.
The prepass writes alpha=0.0 so edge detection ignores holdout geometry.
Material type alias: StandardMaterial + HoldoutExtension.
Invisible occluder that writes depth but outputs fully transparent color.
Use on duplicated geometry in the low-res layer to occlude pixel art entities
behind full-res scene geometry (terrain, walls).