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>

test/t_sim_optimizations.js

@@ -0,0 +1,55 @@
+// Optimized paths must match the full roof scan and the original heat loop exactly.
+const fs = require('fs');
+const { load, suite, flatWorld } = require('./harness');
+const { run } = load();
+const { check, done } = suite();
+run(fs.readFileSync(__dirname + '/fixtures/heat-chunk.js', 'utf8'));
+
+// Partial areas and changes at area boundaries, including material appearing in empty sky.
+flatWorld(run, 99, 97, 80);
+run('RS = 123; bot.x = 45; bot.y = 30;');
+run(`
+  const originalWeatherTick = weatherTick;
+  weatherTick = function () {
+    if (tick % 20 === 1) {
+      const roof = roofY.slice(); updateRoof();
+      if (!roof.every((v, x) => v === roofY[x])) globalThis.__roofMiss = (globalThis.__roofMiss || 0) + 1;
+    }
+    originalWeatherTick();
+  };
+`);
+for (let k = 0; k < 200; k++) run(`
+  setCell((12 + ${k} % 60) * W + (${k} * 31 % W), ${k} % 3 ? WATER : EMPTY, 18, 0);
+  simTick();
+`);
+check('scheduled roof matches a full scan at area edges', !run('globalThis.__roofMiss'), run('globalThis.__roofMiss'));
+run('weatherTick = originalWeatherTick;');
+
+// Every material, threshold neighbours, fractional temperatures, burning, cold neighbours,
+// alternating heat scales, and partial areas exercise both random calls and Float32 stores.
+const snapshot = () => run(`JSON.stringify({
+  arrays: [mat, temp, life, flags, hot, hotLvl, part, digp, seen].map(a => Array.from(new Uint8Array(a.buffer))),
+  RS, stats, EV, brokenMachines
+})`);
+const setup = () => {
+  flatWorld(run, 67, 65, 60);
+  run(`{
+    RS = 456; seen.fill(0); EV.length = 0; stats.ignitions = 0; brokenMachines = [];
+    const ts = [-30, -0.5, 0, 0.5, 18, 97, 100, 260, 380, 600, 880, 1085, 1300, 1530, 1900];
+    for (let i = 0; i < N; i++) {
+      const m = (i * 37) % NMAT;
+      mat[i] = m; temp[i] = ts[i % ts.length] + ((i % 3) - 1) * 0.001;
+      life[i] = i % 256; flags[i] = i % 23 === 0 && FUEL[m] ? F_BURN : 0;
+    }
+    hot.fill(6); hot[0] = 0; hotLvl.fill(2); updateRoof();
+  }`);
+};
+for (const scale of [1, 4]) {
+  setup();
+  run(`for (let c = 1; c < NCH; c++) referenceHeatChunk(c, ${scale});`);
+  const expected = snapshot();
+  setup();
+  run(`for (let c = 1; c < NCH; c++) heatChunk(c, ${scale});`);
+  check('heat matches original bytes and random state at scale ' + scale, snapshot() === expected);
+}
+done();