Resting faces steeper than the angle of repose topple: blown sand towers collapse (t93)

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

docs/DESIGN.md

@@ -570,6 +570,21 @@ above, including:
   Resting heaps do not enter this path; world generation keeps its recorded settling rules
   and seed layouts.
 
+- **Nozzle-built sand towers topple from rest (t93 round 3, 2026-10-28).** The owner
+  reproduced vertical sand towers by continuously blowing straight up. Landing places grains
+  at or above impact; supported face grains need to release before the outer falling column
+  clears. A tall body now spills from a supported exposed face as well as a falling one.
+  Apply this to free-flowing powders (STICK=1); cohesive grains retain their stickier release
+  and do not inherit the outward motion. Require an open side and a steep drop, backed eight grains inward by a continuous column
+  at least 32 grains high (look at most 24 rows each way). This leaves short feeds and the
+  deliberately steeper small clay heaps on their existing settling rules. The spill is still
+  bounded to eight grains horizontally and vertically, checking every crossed cell.
+  Two cell flag bits retain the collapse direction while the released grain falls: up to two
+  grains outward and eight down per tick. Removing support passes that direction to the grain
+  immediately above, so the falling layer drains promptly. Contact below cancels this motion;
+  ordinary repose rules take over. Keep stamped moves, barriers, burning-bed protection and
+  worker independence. The existing worldgen POWDER_FAN switch disables new face releases.
+
 ## Ideas not built yet
 
 - **Concrete**, as a stronger wall material from rubble.