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>

src/sim/weather.js

@@ -71,15 +71,17 @@ function weatherTick() {
       if (sh.left !== undefined) { sh.left -= made; if (sh.left <= 0 && sh.until > tick) sh.until = tick; } // made up the shortfall: it passes
     }
   }
-  // clouds made of your steam build up, then rain it back out where they hang
+  // clouds made of your steam build up, then rain it back out over where they hang and a band
+  // either side, so water boiled off one spot does not all fall back on it
   for (let b = 0; b < n; b++) {
     if (!w.raining[b] && w.moist[b] > 150) w.raining[b] = 1;
-    if (w.raining[b]) { w.moist[b] -= spawnRain(b * WBAND, (b + 1) * WBAND, Math.min(3, 0.5 + w.moist[b] / 100)); if (w.moist[b] < 12) w.raining[b] = 0; }
+    if (w.raining[b]) { w.moist[b] -= spawnRain(Math.max(0, (b - 1) * WBAND), Math.min(W, (b + 2) * WBAND), Math.min(3, 0.5 + w.moist[b] / 100)); if (w.moist[b] < 12) w.raining[b] = 0; }
   }
-  // and drift a little downwind every second; at the edge of the world they pile up instead
+  // and drift a little downwind every second, raining ones at half the pace; at the edge of the
+  // world they pile up instead
   if (tick % 60 === 0) {
     const out = new Float32Array(n), d = w.wind > 0 ? 1 : -1, f = Math.min(0.12, Math.abs(w.wind) * 0.1);
-    for (let b = 0; b < n; b++) { const mv = w.raining[b] ? 0 : w.moist[b] * f; out[b] += w.moist[b] - mv; if (b + d >= 0 && b + d < n) out[b + d] += mv; else out[b] += mv; }
+    for (let b = 0; b < n; b++) { const mv = w.moist[b] * f * (w.raining[b] ? 0.5 : 1); out[b] += w.moist[b] - mv; if (b + d >= 0 && b + d < n) out[b + d] += mv; else out[b] += mv; }
     w.moist = out;
   }
 }