Implement BroadcastChannel for same-origin cross-context messaging #189

Goal#

Implement the BroadcastChannel interface so documents and workers within the same origin can communicate via a named channel, reusing the Phase 19 structured clone and worker message-channel infrastructure.

Depends on: Phase 19 (Web Workers — already complete).

Scope#

• BroadcastChannel interface
  • new BroadcastChannel(name: string) — creates or joins the named channel
  • postMessage(message) — structured-clones the message and dispatches MessageEvent on every other BroadcastChannel with the same name in the same origin
  • close() — removes this channel instance from the registry
  • Properties: name
  • Event handlers: onmessage, onmessageerror plus addEventListener equivalents
• Origin scoping
  • Two BroadcastChannel instances communicate only when they are in browsing contexts (documents or workers) with the same origin
  • Closed channels do not receive or emit
• Cross-thread delivery
  • When the document and a Worker open the same channel, messages cross thread boundaries via the existing worker message-channel infrastructure
  • Use the Phase 19 structured clone implementation (crates/js/src/structured_clone.rs) for serialization
  • Non-cloneable values fire messageerror on receivers (do not abort the sender)
• Registry
  • Per-event-loop registry keyed by (origin, name)
  • Workers and documents register/unregister on construction/close (and on context destruction)
• Lifecycle
  • GC root channels with non-empty onmessage/onmessageerror handlers or pending listeners
  • Closing a context closes all its channels

Acceptance Criteria#

• Two documents in the same origin can exchange messages via the same channel name
• A document and a Worker exchange messages over a BroadcastChannel
• The sender does not receive its own postMessage (per spec)
• name is the value passed to the constructor (or coerced to string)
• After close(), no further messages are received and postMessage throws InvalidStateError
• Non-cloneable values fire messageerror on listening channels, not on the sender
• Origin isolation: a different-origin channel with the same name does not see the messages
• Unit tests covering: same-context dispatch, cross-worker dispatch, sender exclusion, close semantics, origin isolation, structured-clone error path
• Conventions
  • cargo fmt --all --check
  • cargo clippy --workspace -- -D warnings
  • cargo test --workspace