Add client catch-up mode

Increase worker heartbeats per frame when far behind server to fast-forward simulation.
Track backlog and expose catch-up status via TickMetricsEvent.
Extend performance overlay to display backlog turns and indicate active catch-up mode.

src/client/InputHandler.ts

@@ -129,6 +129,10 @@ export class TickMetricsEvent implements GameEvent {
   constructor(
     public readonly tickExecutionDuration?: number,
     public readonly tickDelay?: number,
+    // Number of turns the client is behind the server (if known)
+    public readonly backlogTurns?: number,
+    // Whether the client is currently in catch-up mode
+    public readonly inCatchUpMode?: boolean,
   ) {}
 }