Faster roof scan and heat pass with identical results (t21)

- The roof is rescanned only in area columns that changed since the last scan: a quiet Huge
  world goes from 4.60 to 1.88 ms per scan (every 20 ticks).
- The heat pass skips react() for solids and powders below their first reaction threshold:
  Huge world digging 2.54 -> 2.34 ms/tick, quiet 1.39 -> 1.18.
- Every suite's output is identical apart from timing lines; seeds make the same worlds.
- test/t_sim_optimizations.js checks the dirty roof against a full scan and the heat loop
  against the reference loop byte for byte.

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

src/sim/cells.js

@@ -186,6 +186,7 @@ function simMove() {
       for (let k = 0; k < CW; k++) {
         const cx = ltr ? k : CW - 1 - k;
         if (!awake[rowBase + cx]) continue;
+        roofDirty[cx] = 1;
         const x0 = cx << 5, x1 = Math.min(W, x0 + 32);
         // alternate rows scan left to right and right to left, so nothing drifts one way
         const dx = ltr ? 1 : -1, xEnd = ltr ? x1 : x0 - 1;