Belts carry 4 grains deep; stalled belts warn and stop animating (t45)

docs/DESIGN.md

@@ -142,7 +142,7 @@ above, including:
   short stubs stay for the next firing. The mold cleans itself and rakes the grate after dropping bars or clearing junk.
 - **Screw conveyors 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. If a grain is blocked,
+- **A belt carries a bed four grains deep** (one block, t45), with higher grains left loose. 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,
@@ -447,7 +447,10 @@ above, including:
   2026-10-26).** An overloaded belt line jammed against a screw conveyor above it. The owner
   agreed that's realistic and stays the player's problem, but wants a warning on the stalled
   belt and the animation stopped on everything affected, and belts limited to a realistic
-  load depth instead of carrying 96-grain stacks. Not built yet.
+  load depth instead of carrying 96-grain stacks. Built 2026-10-29: the owner chose
+  **4 grains (one block)**. Loaded transport warns after half a second without progress,
+  with a marker and advice to clear the outlet or heap. Only affected blocks stop animating;
+  a blocked cleated slope stops its whole run. Screens use their slower nudge cadence.
 
 - **Cleats really push their pockets (t42 feedback, 2026-10-26).** The owner found the first
   cleats passed through heavy loads while the material climbed as before, and wants cleats that