`benilla::fog_hook` (fog_linear / fog_sample / apply_fog) replaces the seven
hand-rolled copies of the linear planar-eye-Z fog: terrain, wow_model,
static_gx, liquid, enhanced_water (factor form), wow_effect (scene fog; the
rain streak fade uses fog_linear) and wdl (the hull's own 0..1 yd pair).
Pure functions, values passed as arguments, same expressions in the same
order, so the classic result is bit-identical. `fog_sample` branches on
MonkeyFrame.fog_model (fog3.x) as the FOG lane's extension point, currently
returning the classic result. wow_effect gains a world_pos varying for it.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
