render3d: the shadow array holds only the cascades a fog wall needs, and a thick fog takes the whole sky

A cascade that begins past the wall was fitted and cleared every frame for nothing: the array is now
made with the ones that begin inside it (2 at 30 m, 3 at 175 m, 5 without a fog) and made again when
the wall changes that count; shadow maps 80 -> 32 MB at 30 m, 48 MB at 175 m. A layer the array lacks
clamps to its last, read only past the wall where everything is fogged.

The sky: at walls of 110 m and nearer the whole sky is the fog's colour, overhead too, with a soft
glow at the sun or moon and no stars or clouds through it, easing out by 175 m; from 175 m on the
horizon band alone, and off as before.

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

packages/ludic.render3d/env.ludic

@@ -199,6 +199,7 @@ export state Render3dState {
   gvk_al_tag: []int = null          # per allocation id: owner * 4 + image * 2 + host (vkmem_report.ludic)
   gvk_tag: int = 0                  # the owner allocations are made for now (VKM_*)
   vkmem_at: int = 0                 # R3D_VKMEM=<frame>: the report is printed at that frame
+  sh_layers: int = 5                # the shadow array's layers: the cascades the fog wall needs (shadow_fog)
   fog_imp_released: int = 0         # impostor atlases let go under a fog wall (fog_impostors.ludic)
   fog_imp_rebaked: int = 0
   fog_imp_rebake_us: int = 0