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.

docs/PROJECT_MAP.md

@@ -226,10 +226,10 @@ Exploration only: never changes grains or the random stream.
 - functions: fogAlloc, fogMark, fogSeen, fogInit, fogLook, fogRevealAround, fogRevealAll
 - variables: FOG, FOG_SENSE, FOG_SENSE_FADE, fog, FW, FH
 
-### src/sim/cells.js (176 lines)
+### src/sim/cells.js (187 lines)
 Cell helpers (setCell, move, enter) and the falling-sand movement of powders, liquids and gases (simMove).
 - functions: setCell, swapCells, move, enter, powder, passable, liquid, gas, simMove
-- variables: SORTP
+- variables: SORTP, LSTEP, LFILM
 
 ### src/sim/heat.js (246 lines)
 Heat: conduction, melting and freezing, reactions, fire and air, steel.
@@ -320,7 +320,7 @@ The bot: movement and collision with the world.
 Milestone checks (the ledger) and the sky roof line used for cooling.
 - functions: checkLedger, updateRoof
 
-### src/sim/weather.js (85 lines)
+### src/sim/weather.js (87 lines)
 Weather: clouds from steam, rain and snow, and the world water count.
 - functions: wrnd, resetWeather, rainGap, worldWater, absorbSteam, spawnRain, rainAt, weatherTick
 - variables: WBAND, WS, weather
@@ -329,7 +329,7 @@ Weather: clouds from steam, rain and snow, and the world water count.
 simTick: one simulation step, in order.
 - functions: simTick
 
-### src/sim/worldgen.js (344 lines)
+### src/sim/worldgen.js (346 lines)
 World generation: noise, layers, ores, caves, water, trees, and clearing tiny hard specks.
 - functions: makeNoise, fbm, clearHardSpecks, generateWorld, generateWorldAsync
 - variables: SPECK_MIN
@@ -476,6 +476,7 @@ The frame loop and boot.
 - `test/t_racks.js`: Rack intake, FIFO storage, gate, removal and saved-state validation.
 - `test/t_smooth.py`: The smooth look: the WebGL pass compiles, draws, falls back cleanly, and the thermal lens still works.
 - `test/t_stone.js`: Uses for waste rock and sand: glass bricks from a fan furnace, the Saw, the silo and builders' orders.
+- `test/t_water.js`: Water conservation, settling, ledge pouring, boundaries, pipes and phase changes. Tall worlds keep geothermal heat out of the ponds.
 - `test/t_weather.js`: Weather: no more springs; steam rises into clouds and rains back; showers; snow when it is freezing.
 - `test/t_world.js`: Trees from acorns, charcoal, brick, density, water/lava/snow/ice, full-bag refunds, drill reach, ballistic blowing, snowy mountains.
 - `tools/map.py`: Regenerate the file and symbol index at the end of docs/PROJECT_MAP.md.

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`);