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/ledger.js

@@ -35,7 +35,18 @@ function checkLedger() {
 
 // Topmost non-air cell of each column: air above it can shed heat to the sky.
 function updateRoof() {
-  for (let x = 0; x < W; x++) {
+  scanRoof(0, W);
+  roofDirty.fill(0);
+}
+function updateDirtyRoof() {
+  for (let cx = 0; cx < CW; cx++) {
+    if (!roofDirty[cx]) continue;
+    scanRoof(cx << 5, Math.min(W, (cx + 1) << 5));
+    roofDirty[cx] = 0;
+  }
+}
+function scanRoof(x0, x1) {
+  for (let x = x0; x < x1; x++) {
     let y = 0;
     for (; y < H; y++) { const ph = PH[mat[y * W + x]]; if (ph === PH_SOLID || ph === PH_POWDER || ph === PH_LIQUID) break; }
     roofY[x] = y;