Physics test pass for sand, molten metal and gases; stuck gas finishes, steam is not deleted (t21)

- New test/t_physics.js: heaps, sorting, sinking, world edges; molten copper levelling, setting,
  quenching, slag over copper, cavity filling; smoke, steam, fire, sealed pockets, the top row.
- Gas in a sealed pocket kept its area asleep and never faded or condensed: gas() keeps its area
  awake while it lasts.
- A rain drop or blown grain landing on steam deleted the steam's water: it goes to the clouds.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/sim/cells.js

@@ -126,6 +126,8 @@ function liquid(i, x, y, m) {
   }
 }
 function gas(i, x, y, m) {
+  // Even blocked gas must keep aging or condensing until it is gone.
+  awake[(y >> 5) * CW + (x >> 5)] = 3;
   if (m === FIRE) {
     const l = life[i];
     if (l <= 1) { mat[i] = EMPTY; life[i] = 0; wakeXY(x, y); return; }

src/sim/vacuum.js

@@ -108,7 +108,10 @@ function depositNear(m, T, x, y, skip) {
       const xx = x + dx, yy = y + dy;
       if (xx < 0 || yy < 0 || xx >= W || yy >= H) continue;
       const j = yy * W + xx;
-      if (j !== skip && flyOpen(m, j)) { setCell(j, m, T, 0); return true; }
+      if (j !== skip && flyOpen(m, j)) {
+        if (mat[j] === STEAM) absorbSteam(xx); // displaced steam still belongs to the world's water
+        setCell(j, m, T, 0); return true;
+      }
     }
   }
   return false;
@@ -121,7 +124,10 @@ function landFlyer(f, x, y) {
   for (let k = 0; k < 8; k++) {
     const yy = y - k; if (yy < 0) break;
     const j = yy * W + x;
-    if (flyOpen(f.m, j)) { setCell(j, f.m, f.T, 0); return; }
+    if (flyOpen(f.m, j)) {
+      if (mat[j] === STEAM) absorbSteam(x); // landing replaces the gas, so keep its water in the clouds
+      setCell(j, f.m, f.T, 0); return;
+    }
   }
   if (!depositNear(f.m, f.T, x, y)) { if (f.rain) absorbSteam(x); else bagAdd(f.m, 1, true); } // nowhere to go: back into the bag (rain: back into the clouds)
 }