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/agent-log.md

@@ -4,6 +4,26 @@ 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.
 
+## 2026-10-24: t21 phase 1: water test pass, faster pouring, rain spreads (Claude Code, Codex)
+
+- `test/t_water.js` (20 checks): conservation, levelling and sleep, pools pouring off ledges,
+  world edges and corners, pumps, freezing and thawing, boiling and condensing, a pool draining
+  into a hot shaft.
+- `liquid()`/`move()` in `src/sim/cells.js`: multi-cell steps (`LSTEP`, scaled by `FLUID`),
+  a longer reach for top-layer grains (`LFILM`), and waking the row ahead only when a top-layer
+  grain steps sideways (waking on every liquid move kept 207 of 2048 areas busy after a reload).
+  `worldSteps` settles with both at 1: seeds 777, 77 and 4242 hash the same as before.
+- Found: cooled steam drifting under a roof could stay steam for ever, because its heat area
+  slept. `gas()` now keeps the heat pass on it.
+- `weatherTick()`: raining clouds drift at half pace; rain falls over the band and one either
+  side. `t_weather`: 'close to where it gathered' now samples drops in the air (landed water
+  spreads); a new wind check.
+- Test fixes, not weakening: `t_conveyors` pit check counts steam on its way up (the water total
+  was exact); `t_molds` burning-bed check seeds `RS` (its result hung on earlier tests' draws:
+  0.28-1.04 depending on the sequence, identical on the old and new code).
+- Steady cost after a reload (2048x1024): 1.26 -> 1.24 ms/tick. Tests: `run_all.py --browser`
+  ALL PASSED. Screenshots `shots/t21-pour-120.png`, `shots/t21-pour-900.png`.
+
 ## 2026-10-24: t1: bars stack neatly, Rack blocks store them (Claude Code, Codex)
 
 - `232f5af`: `entTick` snap-and-tip (`entSupportCount`, `src/sim/items.js`);