OptimizedNodeBridge should block late worker spawns after dispose

[bbopen]

Problem

dispose() clears current workers, but spawnProcess() does not check this.disposed. If init() or spawnProcess() is in-flight while dispose() runs, a worker can be added to the pool after disposal and remain orphaned.

Evidence

• src/runtime/optimized-node.ts: dispose() sets this.disposed = true and terminates current pool; spawnProcess() has no guard and always pushes to processPool.

Acceptance criteria

• spawnProcess() should no-op or throw once disposed is true.
• init() should short-circuit when disposed mid-flight.
• Add a test that starts init(), calls dispose() quickly, and verifies no workers remain.

[bbopen]

Resolved by PR #150. BoundedContext blocks operations after dispose by checking state and throwing BridgeDisposedError.