feat(render3d): light you can see - sun shafts and valley mist

Everything before this made the air a COLOUR APPLIED TO A SURFACE. Nothing put light
in the space between surfaces, so the basin had no shafts, no pooled mist and no rays
off a ridge at any hour, whatever was done to the fog.

A half-resolution march from the camera to the depth buffer, asking the same shadow
the rest of the frame asks - the cascades, the baked height-field shadow and the cloud
mask - so a shaft is cast by the actual trees and the actual ridge and a passing cloud
dims its own rays. Henyey-Greenstein scattering, because real air throws light forward.
Density and a separate ground-hugging mist layer ride the sun's elevation, so mist
forms in the cold at either end of the day and burns off by mid-morning.

Composited with the bloom pyramid's own tent upsample under ONE/ONE - what was wanted
and already there - and before bloom, so a shaft blooms. Into post_hdr, not post_scene:
post_scene is what the water refracts and shafts added there would sit under the lake.

The tuning that mattered was the sky term, which is added at every step: at 0.06 it
accumulated into a flat grey wash lifting lit and shadowed air equally, which is the
contrast a shaft is made of, and the valley came out one pale sheet. At 0.012 the sun
dominates and there is light rather than fog. I cut the density and mist three times
before the frame looked like air instead of paint.

R3D_NOVOL=1 for an A/B; Off in Settings skips the pass whole.

400 frames at 07:00: GL 7.1 -> 7.3 s, VK 7.2 -> 7.4 s. Backends agree to 0.08/255.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/ludic.render3d/render.ludic

@@ -285,6 +285,9 @@ function r3d_frame(time: int) -> void {
   post_color = post_hdr.color;  post_color_w = post_w;  post_color_h = post_h
   # DLSS super resolution: the lit frame up to the display's size, before anything reads it
   if gsl_dlss_live() { prof_begin("DLSS");  post_color = gsl_dlss_eval();  prof_end() }
+  # Before bloom and before the tonemap: a shaft of light is a bright thing in the air and
+  # should bloom like one.
+  prof_begin("volumetric");  post_volumetric_pass();  prof_end()
   if not post_no_gi { prof_begin("SSAO/GI");  post_ssao_pass();  prof_end() }
   if r3d_debug_max { tex_max(post_hdr.color, post_hdr.w, post_hdr.h, "hdr") }
   prof_begin("bloom")