render3d: under a fog wall the volumetric air is marched only to it, SSGI stops where it is solid, and card casters that fill the wall stop rebuilding

The volumetric pass marched 800 m in a fixed number of steps whatever the wall: it now stops at the
wall with steps in proportion (at least 8) - 0.57 -> 0.33 ms at 30 m by the pass timers. SSGI skipped
only past 900 m; it now also skips past 0.85 of a wall, where the fog is solid (u_ao_far). applyFog
samples the fog's colour only where its weight is above zero. A card layer whose casters inside the
wall are 80% or more of it (a kilometre's wall) casts from its static list and is not refiltered and
re-uploaded every 4 m of camera motion. Fog off: the frame unchanged (0 pixels over 8).

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

packages/ludic.render3d/scatter.ludic

@@ -39,6 +39,7 @@ property Layer {
   fog_buf:  int = 0,       # the casters inside the fog wall (fog_casters.ludic)
   fog_n:    int = 0,
   fog_src:  int = -1,
+  fog_all:  bool = false,  # the wall keeps (nearly) all of it: casters from sh_buf, no rebuild
   fog_wall: float = 0.0,
   fog_x:    float = 0.0,
   fog_z:    float = 0.0,