fix: Replace busy boolean with inFlightRequests counter in NodeBridge

[bbopen]

Problem

As identified in PR #136 code review, using a boolean busy flag instead of a counter creates a race condition when BridgeCore multiplexes multiple requests on a single worker.

Current Behavior

interface WorkerProcess {
  busy: boolean;  // ← Boolean flag
  // ...
}

private async sendToWorker<T>(worker: WorkerProcess, payload): Promise<T> {
  worker.busy = true;
  // ...
  try {
    return await worker.core.send<T>(payload);
  } finally {
    worker.busy = false;  // ← Sets to false even if other requests in flight
  }
}

Race Condition Scenario:

1. Request A: busy = true
2. Request B (multiplexed on same worker): busy = true (already true, no change)
3. Request A completes: busy = false ← Bug: B still in flight
4. Cleanup sees !busy and may terminate worker with B in flight

Proposed Fix

Replace boolean with counter:

interface WorkerProcess {
  inFlightRequests: number;  // ← Counter
  // ...
}

private async sendToWorker<T>(worker: WorkerProcess, payload): Promise<T> {
  worker.inFlightRequests++;
  // ...
  try {
    return await worker.core.send<T>(payload);
  } finally {
    worker.inFlightRequests--;
  }
}

// In cleanup:
const idleWorkers = this.processPool.filter(
  w => w.inFlightRequests === 0 && /* other conditions */
);

Impact

Low in practice because:

1. Idle cleanup protected: Single-worker mode (default) has minProcesses=1, so processPool.length > minProcesses prevents idle termination of the only worker
2. Overused cleanup rare: Requires 1000+ requests AND the race timing
3. Multiplexing requires concurrent calls: Sequential usage never triggers this

However, this is a correctness bug that should be fixed for multi-worker pools with high concurrency.

References

• PR refactor: unify NodeBridge and OptimizedNodeBridge (ADR-0001) #136 CodeRabbit review comment
• Related to ADR-0001 bridge unification