fix: retain ending streams until worker exit

[mcollina]

Summary

• retain a strong worker-to-stream reference once end() starts
• avoid replacing that reference with a WeakRef when READY races with end()
• run the test suite with --expose-gc and add an inline GC regression test, skipped when GC is unavailable

The worker normally switches to a weak stream reference after startup so abandoned streams can be collected. During end(), however, stream event listeners may be the only remaining references. Because those listeners only form a self-cycle, GC can collect the stream, terminate its worker through the finalization registry, and prevent close from being emitted. This is the timeout seen on Windows/Node 26 in #246 and on main.

Testing

• npm test
• regression test repeated 30 times on Windows with Node 26.8.1
• regression test on Windows with Node 20, 22, and 24