fix construction stuck-at-dirt: snap the shared clock on large drift + tab refocus (was a 0.02s/s crawl, useless after tab throttling)

web/main.js

@@ -159,13 +159,23 @@ function setSim(t, { quiet = false } = {}) {
   return false;
 }
 
-// slew ±2% toward the server clock when drift exceeds 1.5 s — never snap
-setInterval(() => {
+// Keep the sim clock locked to the server clock. serverNowEst() is wall-clock
+// based (accurate even when rAF is throttled); app.simTime is advanced by the
+// render loop and FALLS BEHIND when the tab is backgrounded or the frame rate
+// dips (the dt cap). If it lags, a freshly-built building reads as "not started"
+// and sits at 0% dirt until the clock catches up (June 15 bug). So: gentle slew
+// for tiny drift (cars/day-night don't jump), but SNAP on large drift (after a
+// throttle gap) and whenever the tab regains focus.
+function syncClock() {
   if (!LIVE || anchorPerf == null) return;
-  const drift = serverNowEst() - C.CITY_EPOCH_S - app.simTime;
-  if (Math.abs(drift) <= 1.5) return;
-  setSim(app.simTime + Math.sign(drift) * Math.min(Math.abs(drift), 0.02), { quiet: true });
-}, 1000);
+  const target = serverNowEst() - C.CITY_EPOCH_S;
+  const drift = target - app.simTime;
+  if (Math.abs(drift) <= 1.0) return;
+  if (Math.abs(drift) > 3) setSim(target, { quiet: true });               // snap (post-throttle)
+  else setSim(app.simTime + Math.sign(drift) * 0.25, { quiet: true });    // gentle catch-up
+}
+setInterval(syncClock, 1000);
+document.addEventListener("visibilitychange", () => { if (!document.hidden) syncClock(); });
 
 // ---------------------------------------------------------------- helpers
 const truncate = (s, n) => (s && s.length > n ? `${s.slice(0, n - 1)}…` : s ?? "");