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/shaders/variants.list

@@ -16,6 +16,7 @@ fullscreen.vert|ssao_blur.frag|
 fullscreen.vert|ssgi.frag|
 fullscreen.vert|ternormal.frag|
 fullscreen.vert|tershadow.frag|#define NOISE_ONLY;
+fullscreen.vert|tershadow.frag|#define NOISE_ONLY;#define TS_AUX;
 fullscreen.vert|tonemap.frag|
 fullscreen.vert|tonemap.frag|#define HDR10;
 fullscreen.vert|volumetric.frag|