Clean up previous implementations

removed:
- catchUpMode and its CATCH_UP_ENTER/EXIT thresholds in ClientGameRunner
- tick metrics fields and overlay UI for inCatchUpMode and beatsPerFrame
- leftover worker heartbeat plumbing (message type + WorkerClient.sendHeartbeat) that was no longer used after self-clocking

changed:
- backlog tracking: keep serverTurnHighWater / lastProcessedTick / backlogTurns, but simplify it to just compute backlog and a backlogGrowing flag instead of driving a dedicated catch-up mode
- frame skip: adaptRenderFrequency now only increases renderEveryN when backlog > 0 and still growing; when backlog is stable/shrinking or zero, it decays renderEveryN back toward 1
- render loop: uses the backlog-aware renderEveryN unconditionally (no catch-up flag), and resets skipping completely when backlog reaches 0
- metrics/overlay: TickMetricsEvent now carries backlogTurns and renderEveryN; the performance overlay displays backlog and current “render every N frames” but no longer mentions catch-up or heartbeats

Learnings during branch development leading to this

Once the worker self-clocks, a separate “catch-up mode” and beats-per-frame knob don’t add real control; they just complicate the model.
Backlog is still a valuable signal, but it’s more effective as a quantitative input (backlog size and whether it’s growing) than as a boolean mode toggle.
Frame skipping should be driven by actual backlog pressure plus frame cost: throttle only while backlog is growing and frames are heavy, and automatically relax back to full-rate rendering once the simulation catches up.

src/core/worker/WorkerClient.ts

@@ -100,12 +100,6 @@ export class WorkerClient {
     });
   }
 
-  sendHeartbeat() {
-    this.worker.postMessage({
-      type: "heartbeat",
-    });
-  }
-
   playerProfile(playerID: number): Promise<PlayerProfile> {
     return new Promise((resolve, reject) => {
       if (!this.isInitialized) {

src/core/worker/WorkerMessages.ts

@@ -9,7 +9,6 @@ import { GameUpdateViewData } from "../game/GameUpdates";
 import { ClientID, GameStartInfo, Turn } from "../Schemas";
 
 export type WorkerMessageType =
-  | "heartbeat"
   | "init"
   | "initialized"
   | "turn"
@@ -31,10 +30,6 @@ interface BaseWorkerMessage {
   id?: string;
 }
 
-export interface HeartbeatMessage extends BaseWorkerMessage {
-  type: "heartbeat";
-}
-
 // Messages from main thread to worker
 export interface InitMessage extends BaseWorkerMessage {
   type: "init";
@@ -114,7 +109,6 @@ export interface TransportShipSpawnResultMessage extends BaseWorkerMessage {
 
 // Union types for type safety
 export type MainThreadMessage =
-  | HeartbeatMessage
   | InitMessage
   | TurnMessage
   | PlayerActionsMessage