Offload functions to worker threads with shared memory primitives for Node.js.

refactor(serve): rename Balance → ServerThreadBalancer; pass ServerThread master

The balancer now receives ServerThread[] ({ worker, conns }) and returns the selected thread — disambiguates from moroutine's core Balancer interface and gives custom balancers access to WorkerHandle properties. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>