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/TESTING.md

@@ -78,6 +78,14 @@ checks waking within eight ticks separately from the probabilistic consumption o
   gamepad and world sizes. Screenshots go to
   `shots/play/`. Pixel look by default; `python3 test/play.py shots/play smooth` uses the smooth
   look.
+- `test/t_vacuum_collapse.py` cuts settled stone and mixed heaps with one normal-width
+  vacuum stroke per simulation tick in the real frame loop. It uses the shared
+  `test/pile-cut.js` fixture, requires actual helper dispatch (forced for this small world),
+  checks both faces within 180 ticks, and saves time-series pixel screenshots in `shots/`.
+  `test/t_vacuum_collapse.js` uses the same cuts inline and with Node helpers, including
+  bot-near/far, grain conservation, equal material bytes and forced-awake resting stability.
+  It also checks a broad 120-grain wall within 600 ticks and a spill stopped by cleats
+  across chunk edges, where the source chunk's one-grain barrier flag is clear.
 - `test/t_smooth.py` checks WebGPU smooth shapes, seams, joins, fog and the thermal lens.
 - `test/t_gpu_golden.py` compares 28 cropped frozen reference PNGs (384x288, under
   `test/fixtures/golden/`) against WebGPU: renderer, joins, sky and four light scenes,