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>

changes/light-you-can-see.md

@@ -0,0 +1,32 @@
+bump: minor
+type: feat
+Light you can see: sun shafts, and mist that lies in the valley.
+
+Everything before this made the air a COLOUR APPLIED TO A SURFACE. Nothing put light in
+the space between surfaces, so a basin at dawn had no shafts, no pooled mist and no rays
+off a ridge at any hour, however the fog was tuned.
+
+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, which is
+why a low sun fills a valley when you look into it and does almost nothing when you look
+away. Density and a separate ground-hugging MIST layer ride the sun's elevation from
+daylight.ludic, so the mist forms in the cold at either end of the day and burns off by
+mid-morning.
+
+It composites into the HDR scene with the bloom pyramid's own tent upsample under ONE/ONE
+blending - which is exactly what was wanted and already existed - and before bloom, so a
+shaft blooms like the bright thing in the air it is. Into post_hdr and not post_scene:
+post_scene is the copy the water refracts, and shafts added there would sit under the lake.
+
+The tuning that mattered was the SKY term. It is added at every step, so at 0.06 it
+accumulated into a flat grey wash that lifted lit and shadowed air by the same amount -
+which is the contrast a shaft is made of. The valley came out as one pale sheet with no
+rays in it. At 0.012 the sun's term dominates and there is light to see rather than fog.
+
+R3D_NOVOL=1 switches it off for an A/B. Off in Settings skips the pass whole rather than
+marching once, because a feature is off when it costs nothing.
+
+Measured, 400 frames at 07:00: OpenGL 7.1 -> 7.3 s, Vulkan 7.2 -> 7.4 s. Backends agree
+to 0.08/255.