Loose walls slump to a natural slope instead of eroding layer by layer (t93)

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

docs/DESIGN.md

@@ -548,6 +548,13 @@ above, including:
   and upward escapes when blocked. Grains fall normally after each hop. Dust must show as a few
   soft puffs where grains move; the previous rubble-coloured debris squares were not visible.
 
+- **Loose walls release from their face (t93, 2026-10-28).** The owner wants tall loose
+  rubble faces to collapse quickly toward a pile instead of shedding one surface layer at
+  a time. An open diagonal step with another empty grain below it bypasses the sticky delay;
+  shallow steps retain the material's stickiness. A legal sticky step that misses its chance
+  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.
+
 ## Ideas not built yet
 
 - **Concrete**, as a stronger wall material from rubble.

docs/PROJECT_MAP.md

@@ -254,7 +254,7 @@ Exploration only: never changes grains or the random stream.
 - functions: fogAlloc, fogMark, fogSeen, fogInit, fogLook, fogRevealAround, fogRevealAll
 - variables: FOG, FOG_SENSE, FOG_SENSE_FADE, fog, FW, FH
 
-### src/sim/cells.js (223 lines)
+### src/sim/cells.js (240 lines)
 Cell helpers (setCell, move, enter) and the falling-sand movement of powders, liquids and gases (simMove).
 - functions: setCell, swapCells, swapCellData, move, enter, enterFree, powder, passable, liquid, liquidSearch, gas, wakeBotPowder, simMove, moveChunk
 - variables: SORTP, SORTP_FUEL, LSTEP, LFILM, CORNER_SEAL
@@ -656,6 +656,7 @@ The frame loop and boot.
 - `test/t_nozzle.js`: Blowing aims within the nozzle's reach, with room for the brush and settling.
 - `test/t_perf.js`: Profiling fixtures start in the same state after previous scenes and warm-up ticks.
 - `test/t_physics.js`: Sand and other powders, molten metal and gases: heaps, sorting, sinking, setting, rising and fading. Water has its own suite (t_water.js). Tall worlds keep geothermal heat out of the way.
+- `test/t_pile_collapse.js`: Tall loose faces release together; supported heaps stop moving, including when revisited.
 - `test/t_pipes.js`: Pipes, pumps and outlets move liquids; the Grinder grinds waste rock for trace ore.
 - `test/t_racks.js`: Rack intake, FIFO storage, gate, removal and saved-state validation.
 - `test/t_render_dirty.js`: Render invalidation must not depend on whether a grain moved or the chunk stayed awake.
@@ -709,3 +710,4 @@ The frame loop and boot.
 - `tools/scenes/t40-sloped-belt.js`: Self-contained: a flat feed climbs, crosses the top, and runs downhill; a filter sorts below.
 - `tools/scenes/t88-light.js`: Frozen light comparisons, also used by the WebGPU browser regression suite.
 - `tools/scenes/t88-renderer.js`: Paused comparison scene: ordinary grains, fog, heat, trees and exceptional geometry.
+- `tools/scenes/t93-pile-collapse.js`: Left to right: 40x20 walls of sand, mixed rubble, and rubble with 20% clay. Capture at --ticks 0 and --ticks 180, --zoom 3 --no-light --freeze.

docs/agent-log.md

@@ -5,6 +5,50 @@ note or request it was for, what changed, the files touched, the tests run, and
 open or not verified. Keep each entry short; the git history has the details.
 
 
+## 2026-10-28: t93 loose faces slump without sticky-step stalls (Codex)
+
+- Inspected the owner's screenshot and traced `powder`, `enterFree`, `move`, `moveChunk`,
+  waking and checkerboard worker passes. Movement-only 40x20 walls on a flat floor, with
+  normal sleep countdown and the bot far away: sand reaches a <=1-grain surface step in
+  44 ticks; rubble (stone/copper ore/iron ore/dirt) in 89; 20% clay never reaches it through
+  2400 ticks (2-grain ledge). Always-awake control reaches it in 249 ticks, pure clay in 585;
+  STICK=1 takes 44 for every mix. Cause: probabilistic sticky diagonal steps throttle face
+  release, and three-tick chunk sleep can strand a legal step after missed rolls. Bottom-up
+  scan and stamps still allow sand to collapse; empty destinations are accepted before the
+  target-stamp check. No scheduler or stamp changes were needed.
+- Added a fallback only after a missed sticky roll: an open diagonal with another empty cell
+  below releases immediately; a legal shallow step wakes its chunk to retry. Retained the
+  original fast path, density/displacement rolls, built-surface sliding, sealed corners,
+  occupancy, transport and cleat checks, one-step stamps and per-chunk streams. Burning
+  grains/supports retain their tuned behavior: extending the fallback into burning beds
+  failed vent-fed furnace cleanliness. A shared-loop rewrite was also rejected after profiling.
+- New `t_pile_collapse` has 17 checks: all 800 grains retained; sand, mixed rubble and
+  20% clay faces <=2-grain surface steps by 180 ticks at offsets 0 and 17; cohesive pure
+  clay <=3. After 1200 forced-awake settling ticks, another 600 leave all heap material bytes
+  unchanged and chunks sleep afterward. Settled clay is 27 high versus sand 23. Exact final
+  test against original cells.js fails three collapse checks; final code passes. Added a
+  clay/rubble face spanning chunk rows to the real 0/1/2/8-worker/reversed-pass hash fixture.
+- Final `python3 test/run_all.py --browser` exited 0, ALL PASSED: 33 headless suites,
+  including furnace 9, collapse 17 and threads 12; browser gameplay 361, smooth 28,
+  console 18, caps 7, GPU 15, goldens 28, light 71 and Firefox 2. Evidence:
+  `/tmp/t93-final-browser.log`. Earlier full browser run also passed. Diff whitespace check passes.
+- Final isolated hardware profile, NVIDIA Blackwell, requested command:
+  `python3 tools/perf.py --scenes settled,heat --looks pixel --zooms 6 --compare tools/perf-after-t88.json`.
+  Original-code control -> final total ms/tick: settled 2.0179 -> 1.9790 (-1.9%),
+  heat 2.7255 -> 2.7650 (+1.5%). Against historical t88: 1.8056 -> 1.9790 (+9.6%),
+  2.6231 -> 2.7650 (+5.4%). Final movement costs 0.9715/1.3816 ms/tick versus control
+  0.9951/1.3647. Earlier profiles varied, including unchanged heat work; final run had no
+  concurrent tests. Evidence: `/tmp/t93-perf-fast.json`, `/tmp/t93-idle-control.json`,
+  `/tmp/t93-red-exact.log`, `/tmp/t93-collapse-final.log`.
+- Captured and inspected pixel `shots/t93-before.png` and final `shots/t93-after.png`:
+  three 40x20 walls become sloped heaps after 180 full simulation ticks. Reproduce with
+  `tools/scenes/t93-pile-collapse.js`, `--ticks 0` / `--ticks 180`, `--zoom 3 --no-light --freeze`.
+  No page errors. Rendering code unchanged; smooth screenshot not taken.
+- Files: `src/sim/cells.js`, `test/t_pile_collapse.js`, `test/t_threads.js`,
+  `tools/scenes/t93-pile-collapse.js`, `docs/DESIGN.md`, generated `docs/PROJECT_MAP.md`,
+  t93 records and this log. All subtasks ticked; t93 moved to Review with a completion
+  report and game checks. No commit. Owner playtesting remains open.
+
 ## 2026-10-28: t92 follow-up, visible dust and a botward channel through rubble (Codex)
 
 - Traced shake events through `drainEvents` to `spawnDebris` and `drawParticles`. Events were