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>

tools/scenes/belt-heap.js

@@ -0,0 +1,9 @@
+// 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
+const bx = BX + 6, by = BY;
+for (let x = bx; x < bx + 12; x++) placeBlock(BP_BELT, 1, x, by, true);
+for (let y = by - 10; y <= by; y++) placeBlock(BP_TUBE, 0, bx + 12, y, true);
+for (let y = by * 4 - 30; y < by * 4; y++) for (let x = (bx + 2) * 4; x < (bx + 10) * 4; x++)
+  setCell(y * W + x, y >= by * 4 - 6 ? SAND : CUORE, AMB[y], 0);
+window.CAM = [(bx + 7) * 4, (by - 5) * 4];