Sticky materials slide freely on built surfaces (t61)

Stickiness gated every diagonal step, so clay lumps, snow, wood chunks,
ash and dirt crawled down sloped walls (a clay grain never got down a
20-piece slope). It now applies only grain against grain: a grain
resting on anything built always tries its diagonal step, so chutes
carry every material while heaps keep their shape. The random draw is
unchanged, so generated worlds stay the same.

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

docs/agent-log.md

@@ -4,6 +4,16 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-27: t61 sticky materials slide freely on built surfaces (Claude Code)
+
+- The owner's video showed clay lumps creeping down a sloped wall; removing a piece was
+  unrelated. Cause: STICK gates every diagonal step, so cohesive materials crawl on 45°. In
+  `powder()` (`src/sim/cells.js`) a grain resting on a built solid (not F_NAT) now always tries
+  the diagonal; the random draw is unchanged, so generated worlds are identical.
+- Measured: a clay grain down a 20-piece built slope took 77 ticks, same as sand (before: never
+  arrived in 2000 ticks). Heaps on a built floor and on natural ground are unchanged.
+- Test: `t_slopes.js` clay vs sand transit. `python3 test/run_all.py --browser`: ALL PASSED.
+
 ## 2026-10-27: Unremovable one-cell belt left after cutting slopes (Claude Code)
 
 - The owner's exported save had a lone belt cell no block owned, so nothing could build there or