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; }