Harden gameplay timing and map loading

shared/round-clock.ts

@@ -0,0 +1,19 @@
+// Synchronize at receipt using a bounded estimate of the return journey. The
+// countdown uses performance.now(), so changing the OS clock cannot extend it.
+export function roundClock(
+  serverNow: number,
+  sentAt: number,
+  receivedAt: number,
+) {
+  const serverAtReceipt = serverNow + Math.max(0, receivedAt - sentAt) / 2;
+  return {
+    remaining(deadline: string, now: number) {
+      return Math.max(
+        0,
+        Math.ceil(
+          (Date.parse(deadline) - serverAtReceipt - (now - receivedAt)) / 1000,
+        ),
+      );
+    },
+  };
+}