Web Worker thread infrastructure and message channels #177

Phase 19: Web Workers#

Lay the low-level foundation for Web Workers in the js crate.

Worker thread lifecycle: spawn a dedicated OS thread per Worker, park it on a run-loop waiting for work, and shut it down cleanly on terminate().
Message channel types: MessagePort (a typed wrapper around std::sync::mpsc) and MessageChannel (a pair of connected ports). Both ends must be sendable across threads.
WorkerHandle: an opaque struct held by the main thread that owns the send half of the channel and a JoinHandle. terminate() drops the sender (closing the channel) then joins the thread.
Inbound message queue on the worker thread: the run-loop drains the channel, executing pending messages, and yields back to JS execution when idle.
Thread-safe error propagation: worker-side panics/errors should be forwarded back to the main thread as an ErrorRecord so the JS binding layer can fire onerror.

A unit test spawns a worker thread, sends a string message, receives the echo on the main thread, then terminates cleanly.
No unsafe outside platform/crypto/js (std thread primitives are fine).
cargo test -p we-js passes.