fix: budget workers for expensive captures

packages/engine/src/services/parallelCoordinator.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { distributeFrames } from "./parallelCoordinator.js";
+import { calculateOptimalWorkers, distributeFrames } from "./parallelCoordinator.js";
 
 describe("distributeFrames", () => {
   it("distributes frames evenly across workers", () => {
@@ -42,3 +42,29 @@ describe("distributeFrames", () => {
     expect(tasks.map((t) => t.workerId)).toEqual([0, 1, 2]);
   });
 });
+
+describe("calculateOptimalWorkers", () => {
+  it("lets high-cost auto renders fall back to one worker when CPU budget requires it", () => {
+    const workers = calculateOptimalWorkers(180, undefined, {
+      concurrency: 6,
+      coresPerWorker: 100,
+      minParallelFrames: 120,
+      largeRenderThreshold: 1000,
+      captureCostMultiplier: 4,
+    });
+
+    expect(workers).toBe(1);
+  });
+
+  it("does not apply capture cost to explicit worker requests", () => {
+    const workers = calculateOptimalWorkers(180, 4, {
+      concurrency: 6,
+      coresPerWorker: 100,
+      minParallelFrames: 120,
+      largeRenderThreshold: 1000,
+      captureCostMultiplier: 4,
+    });
+
+    expect(workers).toBe(4);
+  });
+});