fix: load native addon in worker_threads

[enochgroot]

Summary

Fixes msgpack/msgpack-node#60: requiring the native addon in the main thread then in a worker_threads Worker threw Error: Module did not self-register.

• Register with NODE_MODULE_CONTEXT_AWARE instead of NODE_MODULE.
• Keep the sbuffer pool, unpack.bytes_remaining, and cycle-detection key thread_local so workers do not race the main isolate.

Test plan

• npm test — 57/57 pass (includes 4 new worker_threads tests)
• Repro: require() in main, then in a Worker — no longer throws
• Worker pack/unpack of {a:1, b: Buffer.from('hi')}
• Cycle detection still throws inside a worker
• Main-thread binding.bytesRemaining() is not clobbered by a worker unpack

[enochgroot]

Review round 2 (f2236a0 on fix/worker-threads):

• sbuf_pool is now static thread_local SbufPool sbuf_pool;
• Module register is NAN_MODULE_WORKER_ENABLED(msgpackBinding, Init) (the NODE_MODULE_CONTEXT_AWARE function-pointer warning is gone)
• ~PackBuffer offers every sbuffer back to the thread's pool unless it is full, so the pool actually fills
• Concurrent pack test: 4 workers × 4000 round-trips overlapping the main thread; sequential 2000-pack pool-reuse test for CopyBuffer vs NewBuffer

npm test: 57 pass / 0 fail (Node 20.20.2 linux/arm64). Rebuild clean.

Does not open a PR against msgpack/msgpack-node.