Falling grains 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 (103 lines)
+### src/render/light.js (104 lines)
 Propagated coloured light from glowing and burning cells.
-- functions: builtForLight, onBeltForLight, computeLight
+- functions: builtForLight, looseForLight, 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: falling grains cast no shadows (Claude Code, Codex)
+
+- The owner saw a single falling grain darken a column of sky. `onBeltForLight` became
+  `looseForLight` (`src/render/light.js`): a loose stack that ends in air or gas within 12 cells
+  (falling) is see-through, as are belt and filter loads; stacks resting on any solid or a tube
+  part still shade.
+- `test/t_light.js`; `run_all.py --browser` ALL PASSED.
+
 ## 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