Grains riding a belt or filter cast no shadow (t37)

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

docs/PROJECT_MAP.md

@@ -352,9 +352,9 @@ Drawing (src/render/): browser only. This file: the renderer state R, colour tab
 - functions: glowIndex, thermIdx, pipeCellColor, pipeBodyCell, thinPipeColor, beltLike, buildSky, initRender, resizeRender, viewCells, updateCamera, snapCamera, screenToWorld, worldToScreenPx, renderMonoFont
 - variables: R, GLOW_W, GLOW_R, GLOW_G, GLOW_B, THERM, PC, TREE_R, TREE_G, TREE_B, BG, SKY_R, SKY_G, SKY_B
 
-### src/render/light.js (95 lines)
+### src/render/light.js (103 lines)
 Propagated coloured light from glowing and burning cells.
-- functions: builtForLight, computeLight
+- functions: builtForLight, onBeltForLight, computeLight
 
 ### src/render/world.js (332 lines)
 Scale colour without changing the smooth-look class.

docs/agent-log.md

@@ -4,6 +4,14 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-25: t37 follow-up: belt loads cast no shadows (Claude Code, Codex)
+
+- The owner saw grains riding a belt overhead still shading the ground. `onBeltForLight(i)` in
+  `src/render/light.js` walks down through up to 12 loose cells to a belt or filter; such cells
+  are skipped by the light roof and pass light like air. `builtForLight` is unchanged, so the t33
+  loose-grain shading still applies to belt loads. Heaps on rock, silos and wall floors still shade.
+- `test/t_light.js` (56 checks); `run_all.py --browser` ALL PASSED.
+
 ## 2026-10-25: t35: diagonal screw tubes (Claude Code, Codex)
 
 - Parts `P_TUR`..`P_TUL` (11-14), `TUBE_PARTS`, and an `IS_TUBE` table replacing every