Piles near the bot sort, and 20 times faster; burning fuel keeps the slow pace (t21)

- SORTP 0.003 -> 0.06: ore sinks through 8 rows of ash in about 7 s near the bot.
- wakeBotPowder(): every 8 ticks, areas with loose grains within 100 grains of the bot get a
  sorting chance; far away, resting piles still sleep.
- Sinking through burning fuel stays at 0.003, so buried burning piles still char and furnace
  beds behave as tuned. World generation settles at the old rate: seeds are unchanged.

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

docs/DESIGN.md

@@ -351,6 +351,13 @@ above, including:
   - World generation settles the land at the old water speed, so every seed keeps making the
     same world.
 
+- **Piles near the player sort, and quickly (t21, 2026-10-24).** A grain at least twice as heavy as
+  the powder under it sinks through it at 20 times the old pace (ore through ash: 4 rows to the
+  bottom in about 7 s). Before, a resting pile went to sleep and never sorted. Now areas with loose
+  grains within about 100 grains of the bot get a sorting chance every 8 ticks; far away, piles
+  still sleep. Through burning fuel sinking stays at the old slow pace: furnace beds are tuned to
+  it, and dirt heaped on a burning pile must smother it into charcoal rather than sink through.
+
 ## Ideas not built yet
 
 - **Concrete**, as a stronger wall material from rubble.