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/PROJECT_MAP.md

@@ -248,8 +248,8 @@ Pipes, pumps and outlets: each joined line holds one liquid as a count, not as g
 Per-tick machine work: steam engines, fans, belts, filters, screw tubes (machinesTick).
 - functions: engineIntake, fanBlow, machinesTick
 
-### src/sim/conveyors.js (109 lines)
-How belts, filters and screw tubes move what is on or in them. What rests on a belt or filter rides along: the whole stack standing on the carrying surface steps one cell toward d together (belts run front of the line first, so the way ahead is clear), the way a real belt carries a heap, not just its bottom layer. A filter's own material drops through it instead, unless the filter below is already full.
+### src/sim/conveyors.js (114 lines)
+How belts, filters and screw tubes move what is on or in them. What rests on a belt or filter rides along: the whole stack standing on the carrying surface steps one cell toward d together (belts run front of the line first, so the way ahead is clear). If a powder grain is blocked, the load above it stands still and the belt slips underneath. A filter's own material drops through it instead, unless it is already full.
 - functions: carryColumn, beltDrain, launchGrain, rebuildFilters, filterCarry, setFilter, filterRun, roomFor, tubeIntake
 - variables: CARRY_DEPTH, filterR, filterL, filtersDirty
 
@@ -257,10 +257,10 @@ How belts, filters and screw tubes move what is on or in them. What rests on a b
 Building blocks: placing, integrity, removing, lines and rectangles of blocks.
 - functions: blockOf, blockCellList, pieceCost, canPlaceBlock, placeBlock, blockIntegrity, removeBlock, blockLine, blockRect, blockInReach, blockIs, placeBlocks
 
-### src/sim/molds.js (297 lines)
+### src/sim/molds.js (298 lines)
 Molds: cavities for castings; ingot troughs drop bars out of the bottom.
 - functions: moldCellsAt, canPlaceMold, newMoldRecord, addMold, castMix, cavityState, troughState, clearCavity, moldPlugCells, rakeAbove, furnaceJunkHint, unplugMold, ejectIngot, checkMolds, moldAt, knockNote, tally, collectMold, removeMold
-- variables: DROSS, grains
+- variables: DROSS, grains, barWord
 
 ### src/sim/items.js (134 lines)
 Bars and other loose objects (ents), and the bag: adding, taking, costs and refunds.
@@ -467,5 +467,6 @@ The frame loop and boot.
 - `tools/experiments/smelter.js`: Smelter design experiments, built with the same block pieces a player uses. Layout (block rows, 4x4 grains each), on flat ground at row 25: roof with a vent ........ row g-CH-1 chamber, CH blocks tall .. charge of coal + copper ore inside grate floor ............. row g = 23 (liquids pass, powders do not) ingot mold .............. row 24, sitting on the ground side walls run from the roof down past the mold. Usage: node t_smelter.js [key=value ...]
 - `tools/experiments/starter.js`: Plays a starter design the way a player would: paste the blueprint, let the bot build it from the bag, blow an even ore/coal mix inside, light it, wait, knock out the castings. Usage: node tools/experiments/starter.js [design id] [ore] [coal]
 - `tools/experiments/worldgen.js`: World generation report: time to generate, how many cells of each material, where the bot starts, and what idle ticks and ticks after digging cost.
+- `tools/scenes/belt-heap.js`: A belt feeding an upward screw: sand underneath a deep heap of copper ore. Run clear-sky.js first, e.g. python3 tools/shot.py /tmp/belt-heap.png --setup tools/scenes/clear-sky.js --js "$(cat tools/scenes/belt-heap.js)" --ticks 240 --zoom 7
 - `tools/scenes/clear-sky.js`: A clear work area left of the bot's starting point (the trade dock is to its right): open sky 160 grains wide and 40 tall, over a flat stone floor. Money and a bag of building stuff to build with. Sets FLOOR (the first row of the floor), BX, BY (the leftmost block of the area that stands on the floor) and CAM (the middle of the area).
 - `tools/scenes/stone-uses.js`: The stone and sand machines: a half-full silo with its gate open over a belt, a stone cutter fed with rubble, and cut stone and glass brick walls. Run clear-sky.js first, e.g. python3 tools/shot.py /tmp/stone.png --setup tools/scenes/clear-sky.js --js "$(cat tools/scenes/stone-uses.js)" --ticks 240 --zoom 7