PSX surface shader

Adapted from installed COINFALL game.reuse.ps1. Provides 320x240 clip-space vertex snapping, affine UVs, quantized vertex lighting and distance fog.

texture_resolution limits the longest texture axis to a power-of-two sampling grid; default 128. Lucia and Jason use 256 for their packed full-body atlases. Original image assets are preserved. The shader uses a reduced mip where available, otherwise a 4x4 stratified box approximation within each virtual texel, then nearest enlargement. Small textures retain their native resolution. This texture-space effect remains visible without the separate screen post-process.