Worker now self-clocks; no heartbeats needed

GameRunner exposes pending work via a new hasPendingTurns() so the worker can check whether more ticks need to be processed.
Worker auto-runs ticks: as soon as it initializes or receives a new turn, it calls processPendingTurns() and loops executeNextTick() while hasPendingTurns() is true. No more "heartbeat" message type; the worker no longer depends on the main thread’s RAF loop to advance the simulation.
Client main thread simplified:
Removed CATCH_UP_HEARTBEATS_PER_FRAME, the heartbeat loop, and the lastBeatsPerFrame tracking.
keepWorkerAlive now just manages frame skipping + draining. When it decides to render (based on renderEveryN), it drains pendingUpdates, merges them, updates GameView, and runs renderer.tick().
Because rendering a batch always implies draining, we restored the invariant that every GameView.update is paired with a layer tick() (no more lost incremental updates).
MAX_RENDER_EVERY_N is now 5 to keep the queue from growing too large while the worker sprints.

src/client/graphics/layers/PerformanceOverlay.ts

@@ -435,7 +435,7 @@ export class PerformanceOverlay extends LitElement implements Layer {
   private renderEveryN: number = 1;
 
   @state()
-  private beatsPerFrame: number = 1;
+  private beatsPerFrame: number | null = null;
 
   updateTickMetrics(
     tickExecutionDuration?: number,
@@ -491,7 +491,7 @@ export class PerformanceOverlay extends LitElement implements Layer {
       this.renderEveryN = renderEveryN;
     }
     if (beatsPerFrame !== undefined) {
-      this.beatsPerFrame = beatsPerFrame;
+      this.beatsPerFrame = beatsPerFrame ?? null;
     }
 
     this.requestUpdate();
@@ -647,7 +647,8 @@ export class PerformanceOverlay extends LitElement implements Layer {
         ${this.inCatchUpMode
           ? html`<div class="performance-line">
               Render every <span>${this.renderEveryN}</span> frame(s),
-              heartbeats per frame: <span>${this.beatsPerFrame}</span>
+              heartbeats per frame:
+              <span>${this.beatsPerFrame ?? "auto"}</span>
             </div>`
           : html``}
         ${this.layerBreakdown.length