Faces cut into loose piles collapse outward instead of draining column by column (t93)

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

docs/DESIGN.md

@@ -555,6 +555,21 @@ above, including:
   keeps its chunk awake to retry. Burning fuel beds keep their tuned settling behavior.
   Grain stamps, sealed corners, transport barriers and per-chunk random streams still apply.
 
+
+- **Vacuum-cut loose faces spread while falling (t93 reopened, 2026-10-28).** The owner
+  reports that stone-only and mixed faces still stand after vacuuming into settled piles.
+  Sticky-step retries alone do not address the descending column formed by always choosing
+  straight down first. A falling grain on an exposed vertical face, backed by supported
+  powder, prefers an open outward diagonal. With at least 12 grains of backing above it
+  and unmoved support below that backing, it spills across up to eight clear grains and
+  falls up to eight clear grains. This is a local pressure approximation: the falling
+  outer column must drain quickly enough to let the body of the pile release. Shallow
+  faces keep the one-grain step, preserving small heaps and conveyor feeds. Check the
+  whole horizontal/downward path for occupancy, parts and cleats, including other chunks.
+  Keep one stamped move per tick and the existing eight-grain worker write halo.
+  Resting heaps do not enter this path; world generation keeps its recorded settling rules
+  and seed layouts.
+
 ## Ideas not built yet
 
 - **Concrete**, as a stronger wall material from rubble.