Belts: a backed-up heap stands still instead of the top sliding over the bottom

carryColumn stops at the first blocked grain on a belt, so a heap whose front
is backed up (a screw tube taking less than the belt brings, or a wall) no
longer shears. New t_conveyors checks and a belt-heap screenshot scene.

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

docs/agent-log.md

@@ -4,6 +4,26 @@ 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.
 
+## 2026-10-23: a backed-up heap on a belt no longer shears (Claude Code, with Codex)
+
+Note: "belts with a big pile on top only move the grains on the top of the stack…". The owner's
+setup: a belt ending in a screw tube going up; material mixed into the bottom layers never moved.
+
+- Cause: `carryColumn` (`src/sim/conveyors.js`) skipped a blocked grain and carried on up the
+  column, so with the front backed up the top slid over a stuck bottom. Reproduced headlessly
+  first: over 4,000 ticks, 8,815 shear steps at the tube (1,415 against a wall) and only 57 of
+  192 bottom sand grains left the belt.
+- Fix: on a belt, the walk stops at the first blocked powder grain, so the load above stands
+  still. On a filter the walk goes on only so chosen materials can still drop through; nothing
+  above a blocked grain is carried. After: no shear, 119 of 192 sand grains out through the tube.
+- `test/t_conveyors.js`: 3 new checks (no shear into an up tube, none against a wall, bottom
+  layer leaves through the tube). `tools/scenes/belt-heap.js` sets up the owner's case for
+  `tools/shot.py`; shots at 30 and 900 ticks look right. `docs/DESIGN.md`, `docs/PROJECT_MAP.md`.
+- Tests: all 11 headless suites and `play.py` (109) pass. `t_smooth.py` failed its
+  half-resolution check once at load 6.5 and passed on the rerun.
+- Open: `t_smooth.py`'s half-resolution check is flaky on a busy machine (the game drops to
+  pixel when frames are slow). It is worth making that test immune to the slow fallback.
+
 ## 2026-10-23: ingots drop out of the bottom of the mold (Claude Code, with Codex)
 
 Note: "copper bars go into the furnace. also they shouldnt require the player to press F…".