Profiler: real-GPU breakdown by zoom, look and scene, with a baseline (t88 phase 0)

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

src/ui/perf.js

@@ -0,0 +1,15 @@
+// Browser-only clocks; simulation code stays usable without a browser or a clock.
+let PERF_MARKS = false, PERF_FRAME = null;
+const perfMarks = {}, perfTimes = {}, perfLast = {};
+for (const name of ['drawWorld', 'computeLight', 'simMove', 'heatPass']) {
+  const original = window[name];
+  window[name] = function (...args) {
+    if (!SET.showPerf && !PERF_MARKS) return original.apply(this, args);
+    const start = performance.now();
+    const result = original.apply(this, args);
+    const ms = performance.now() - start;
+    perfLast[name] = ms;
+    if (name !== 'computeLight') perfTimes[name] = (perfTimes[name] || 0) * 0.9 + ms * 0.1;
+    return result;
+  };
+}