[Story 1.2] Add TaskSemaphore utility #362

Part of #356 (Epic 1: Foundation).

Depends on: Nothing — additive, no behavior change.

Context

No reusable concurrency primitive exists for limiting concurrent tasks with introspectable queue state. async-mutex is already in deps at ^0.5.0 and provides a battle-tested Semaphore, but it doesn't expose a waiting count getter needed for TaskScheduler observability and tests. A thin wrapper is the right call here — ~10 lines vs building a custom semaphore from scratch.

Developer Notes

Create src/utils/TaskSemaphore.ts:

import { Semaphore } from 'async-mutex'

export class TaskSemaphore {
  private sem: Semaphore
  private _waiting = 0

  constructor(permits: number) {
    this.sem = new Semaphore(permits)
  }

  get available(): number { return this.sem.getValue() }
  get waiting(): number { return this._waiting }

  async acquire(): Promise<() => void> {
    this._waiting++
    try {
      const [, release] = await this.sem.acquire()
      this._waiting--
      return release
    } catch (e) {
      this._waiting--
      throw e
    }
  }

  cancel(): void { this.sem.cancel() }
}

cancel() rejects all queued acquire() waiters — used by TaskScheduler.cancelAll() and dispose().

Files: src/utils/TaskSemaphore.ts (new)

Tests (src/utils/__tests__/TaskSemaphore.spec.ts):

• acquire() resolves immediately when permits are available.
• Second acquire() queues when no permits remain; resolves after release.
• Returned release function restores exactly one permit and unblocks one waiter.
• available and waiting return correct values at each step.
• cancel() rejects all queued waiters.
• cancel() resets waiting count to 0 (no counter drift from rejected acquirers).
• Use vi.useFakeTimers() — no real setTimeout.

Acceptance Criteria

• No circular imports into src/core/.
• All tests complete in < 50ms.