Water pours off ledges several times faster, raining clouds spread boiled-off water (t21)

- Liquids step up to 8 cells a tick toward a drop (fewer the thicker they are), and a top-layer
  grain looks 4 times as far for one, so pools empty and thin film drains. A 100-wide pool
  empties in about 50 s; the old code left 24-650 grains stranded on wide ledges.
- Only a top-layer grain stepping sideways wakes the row ahead, so settled lakes still sleep.
- World generation settles at the old speed: seeds make the same worlds as before.
- Cooled steam under a roof no longer stays steam for ever (its heat area slept).
- Raining clouds keep drifting at half pace and rain over a band either side, so a waterfall into
  a hot shaft no longer rains straight back on its own lake.
- New test/t_water.js; t_weather rain-location checks; t_conveyors counts steam in flight;
  t_molds seeds the random burning-bed check.

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

docs/DESIGN.md

@@ -338,6 +338,19 @@ above, including:
   version's narrow fade too sharp.
   Unexplored ground is pure black, not dark grey (the owner: "should be black anyway").
 
+- **Water pours faster, and boiled-off rain spreads out (t21, 2026-10-24).** The owner found water
+  running off a ledge slower than expected. Only the top layer of a pool flows, so pools emptied
+  at a trickle and the old code left 24-650 grains stranded on a wide ledge for good.
+  - A liquid now steps up to 8 cells a tick toward a drop instead of 1, fewer the thicker it is
+    (water 8, molten copper 5, slag 4, magma 2, glass 1). A grain on top may look 4 times as far
+    for a drop, so thin film left behind drains too. A 100-wide pool empties in about 50 s
+    (it never fully emptied before); settled lakes still sleep.
+  - Raining clouds keep drifting downwind at half pace, and rain falls over a cloud's band and the
+    band either side. Before, a waterfall into a shaft hot enough to boil it could rain straight
+    back on its own lake.
+  - World generation settles the land at the old water speed, so every seed keeps making the
+    same world.
+
 ## Ideas not built yet
 
 - **Concrete**, as a stronger wall material from rubble.