Grains resting on anything built cast no shadow (t37)

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

docs/PROJECT_MAP.md

@@ -352,7 +352,7 @@ 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 (104 lines)
+### src/render/light.js (103 lines)
 Propagated coloured light from glowing and burning cells.
 - functions: builtForLight, looseForLight, computeLight
 

docs/agent-log.md

@@ -4,6 +4,15 @@ 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: grains on built things cast no shadows (Claude Code, Codex)
+
+- The owner saw a flickering shadow under a grinder fed by a stream: grains resting on its
+  housing for a moment shaded. `looseForLight` (`src/render/light.js`) now treats a loose stack
+  that rests on any `builtForLight` cell, or ends in air or gas, as see-through. Stacks on natural
+  ground or deeper than 12 cells still shade.
+- `test/t_light.js` (a grinder stream checked for 180 ticks in both lighting modes);
+  `run_all.py --browser` ALL PASSED.
+
 ## 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