render3d: the survey DEM is let go once the height field is made, and the terrain sun shadow is R32F + RG16F

The DEM (R16 with mips, 44 MB) was read only by terrain_generate and kept for the map's life; a
world swap loads its own again. The height-field sun shadow was one RGBA32F for three values and an
unused fourth: the lowest lit height stays 32-bit (a receiver 3000 m up compares against it to a
quarter metre), the occluder distance and the cloud mask go beside it in RG16F (64 -> 32 MB), baked
in a pass of their own (TS_AUX): a pipeline takes one colour format for all its targets, and drawn
together into R32F + RG16F the writes went nowhere and nothing was lit.

-75 MB at every fog level, off included (2008 -> 1933 MB at the overlook); the fog-off frame is
unchanged (0 pixels over 8).

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

packages/ludic.render3d/env.ludic

@@ -828,7 +828,9 @@ export state Render3dState {
   ter_dem_base: float = 0.0
   ter_ortho_tex: int = 0            # a photograph of the same window, draped with distance
   ter_carpet: int = 0               # the distant-grass carpet (carpet_bake), 0 = none
-  ter_shadow_tex: int = 0           # height-field sun shadow: RG32F (lowest lit height, occluder distance)
+  ter_shadow_tex: int = 0           # height-field sun shadow: R32F, the lowest lit height
+  ter_shadow_prog_aux: int = 0      # the bake of the aux target (TS_AUX)
+  ter_shadow_aux: int = 0           # beside it, RG16F: the occluder distance and the cloud mask
   ter_shadow_yaw: float = 1000000000.0  # the sky yaw it was baked for
   ter_shadow_gen: int = -1          # the daylight generation it was baked for
   ter_shadow_prog: int = 0