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>

test/t_molds.js

@@ -94,8 +94,9 @@ run('for (let x = 100; x < 104; x++) setCell(98 * W + x, ASH, 1320, 0); for (let
 run('for (let k = 0; k < 120; k++) { for (let y = 90; y < 100; y++) for (let x = 96; x < 108; x++) if (mat[y * W + x] === ASH) temp[y * W + x] = Math.max(temp[y * W + x], 1320); simTick(); }');
 check('ash fuses into slag above 1250°C', count(run, run('ASH'), 96, 90, 107, 99) === 0 && count(run, run('MSLAG')) + count(run, run('SLAG')) === 4, `ash ${count(run, run('ASH'), 96, 90, 107, 99)}, slag ${count(run, run('MSLAG')) + count(run, run('SLAG'))}`);
 check('ash in a cooler fire stays ash', count(run, run('ASH'), 108, 90, 116, 99) === 4);
-// a mixed charge stays mixed until it burns; ore then works its way down through the burning bed
-fresh();
+// a mixed charge stays mixed until it burns; ore then works its way down through the burning bed.
+// The sinking is random: seed it so the result does not hang on what earlier tests drew.
+fresh(); run('RS = 7919;');
 rect(run, 100, 90, 101, 99, run('STONE')); rect(run, 114, 90, 115, 99, run('STONE'));
 run('{ const r = mulberry32(3); for (let y = 90; y <= 99; y++) for (let x = 102; x <= 113; x++) setCell(y * W + x, r() < 0.5 ? CUORE : COAL, AMB[y], 0); }');
 const oreY = () => run('(()=>{let s=0,n=0;for(let i=0;i<N;i++) if(mat[i]===CUORE){s+=(i/W)|0;n++;} return s/n})()');