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/DESIGN.md

@@ -471,6 +471,12 @@ above, including:
   but make it about what's on screen, so anything the player can see and aim at builds directly
   and anything beyond stays a blueprint built when the bot comes near. No limit at all would let
   players build in places they've never been and link far claims instantly. Not built yet.
+- **Sticky materials slide freely on built surfaces (t61, 2026-10-27).** Stickiness (STICK) is the
+  chance per tick a grain takes a diagonal step, which made clay lumps, snow, wood chunks, ash and
+  dirt creep down sloped walls (a single clay grain never got down a 20-piece slope). The owner
+  chose to make stickiness grain against grain only: on anything built, every material slides
+  freely, so slopes are useful for every material, while heaps keep their shape. Real chutes clog
+  with sticky material and need cleaning, but that is micromanagement the game doesn't need.
 
 ## Ideas not built yet