MaterialAlphaMode enum

Kept or dropped per pixel against noise instead of a threshold — gfx-16n's own row, and the one mode here that glTF has no word for.

What it is for: foliage and nets. A leaf texture at 40% opacity is either entirely there or entirely gone under mask, so a fern comes out as a hard-edged cut-out; blend draws it correctly and needs the geometry sorted, which costs a sort per frame and defeats instancing. Hashed keeps 40% of the pixels and resolves as 40% opacity to anything that averages several of them.

It is drawn in the opaque half, writes depth, and needs no sorting — which is the whole point — at the price of visible noise anywhere the result is not averaged down. Without temporal accumulation this engine does not have, that price is real: it suits a supersampled render or a distant canopy better than a leaf held up to the camera.

The noise is anchored to world position rather than to the screen, so a moving branch keeps its verdict instead of sparkling as it passes through a fixed pattern. See surface.glsl, which does the work.