Web Worker Pool — CPU-Intensive Tasks

## Task
Web Worker pool for distributing CPU-intensive work across browser threads.

## Requirements
- TypeScript
- No external dependencies
- Works in all modern browsers

## API
```typescript
const pool = new WorkerPool({
  workerUrl: "/workers/processor.js",
  poolSize: navigator.hardwareConcurrency || 4,
  taskTimeout: 30_000, // 30s max per task
});

// Submit tasks
const result = await pool.exec("processImage", {
  imageData: arrayBuffer,
  filters: ["sharpen", "resize"],
});

// Submit with progress
const result = await pool.exec("trainModel", data, {
  onProgress: (pct) => setProgress(pct), // 0-100
});

// Batch processing
const results = await pool.map(items, "processItem");

// Cleanup
pool.terminate();
```

## Implementation Notes
1. Round-robin task assignment with busy tracking
2. Transfer ArrayBuffers (zero-copy) for large data
3. Structured clone for regular objects
4. Timeout handling: kill worker and spawn replacement
5. Error isolation: one worker crash doesn't affect others
6. Idle timeout: terminate unused workers after 60s, respawn on demand
7. Progress: worker posts incremental messages, pool forwards to caller