fog post-process shader

Depth-keyed distance haze. pp.add(name, "fog", { density = ..., color = { r, g, b } }) registers it, and asset.resolve("@builtin::shaders.post.fog", "shader"):getProperties() reads the knobs below before it is added.

The pass reads the scene depth already drawn at a pixel, turns it into a distance in metres with zero_linear_depth(d, near, far), and mixes the frame toward fogColor by 1 - exp(-distance * density) — an exponential falloff, so density sets how many metres of air it takes to reach the fog colour.

near and far are the planes the frame was projected through. They are what a stored depth is turned back into metres by, so they belong to the camera the scene was drawn with rather than to the look: setting them to something the camera does not use moves every surface's distance and with it the whole haze.

Property	Meaning
density	Falloff rate. 0 leaves the frame untouched.
fogColor	The colour a fully fogged pixel reaches.
near	The scene camera's near plane, in metres.
far	The scene camera's far plane, in metres.