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/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.