render3d: r3d_fog_wall(dist) - fog that closes to the sky's horizon colour from 0.3 dist to dist, and nothing drawn past dist + 8 m

Every lit program blends toward the horizon colour (the prefiltered sky with the sun's inscatter,
so it carries the day's grade); the sky's horizon band is pulled to the same colour, wider the
closer the wall. The far plane, scatter and stream cull reach and the shadow cascades are clamped
to the wall; the reflection pass shares the shaders and the cull. 0 is off: every branch is gated
on u_fog_wall > 0 and r3d_reach hands back the far it was given. R3D_FOG_WALL=<m> for a shot.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.render3d/env.ludic

@@ -584,6 +584,7 @@ export state Render3dState {
   q_sz: floats = null
   q_st: floats = null
   r3d_sky_prog: int = 0
+  r3d_fog_wall: float = 0.0          # r3d_fog_wall: > 0, nothing past it is seen or drawn
   r3d_fog_density: float = 0.0
   r3d_fog_falloff: float = 0.0
   r3d_fog_inscatter: float = 0.02   # 0.02, what the shader used to hard-code