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/DESIGN.md

@@ -140,8 +140,9 @@ above, including:
   short stubs stay for the next firing. F stays available for cleaning and raking.
 - **Screw tubes carry no more than belts:** they move every other tick. For more throughput,
   build more lines.
-- **A heap on a belt moves as one**, up to 96 grains deep, not just its bottom layer. The owner
-  reports this still misbehaves (see NOTES.md).
+- **A heap on a belt moves as one**, up to 96 grains deep, not just its bottom layer. If a grain is blocked,
+  the load above it stands still and the belt slips underneath; the top no longer slides over
+  a stuck bottom. Filters still let their chosen materials drop through.
 - **Pick** (middle mouse) selects the piece under the cursor with its direction,
   wall material and filter settings. G is unbound, as the owner requested.
 - **The build menu** is a bar of tabs and icon tiles along the bottom: Structure, Moving,