Add async observable scaffolding

[rchatham]

Summary

Begins Swift concurrency modernization for the observer pipeline by adding actor-backed async observable scaffolding while keeping the live synchronous event pipeline unchanged.

Changes

• Add internal AsyncObservable<T> actor for async value/observer storage and delivery.
• Add internal AsyncMultiObservable<T> actor for keyed async observer storage and delivery.
• Add SyncObservableBridge helpers that run async observable operations and block the current synchronous delegate/GCD thread until completion.
• Document sync-over-async caveats: use only from synchronous entry points; prefer await; do not call from actor-isolated/cooperative executor contexts.
• Add async and sync-bridge tests covering:
  • current-value replay when observers are added
  • registration-order delivery for AsyncObservable
  • concurrent async updates
  • async keyed observer removal
  • synchronous bridge waiting for delivery completion

Verification

• swift test --filter AsyncObservableTests
• swift test
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug

Notes

• This is intended as a stacked follow-up on PR Serialize event observer mutations #24 (feature/network-migration-threadsafe-events).
• Existing Observable, MultiObservable, and PeerConnectionResponder runtime behavior is not rewired in this PR.
• The synchronous bridge is intentionally internal scaffolding for future migration steps that need to preserve synchronous callback semantics while moving storage to actors.

[rchatham]

Superseded by consolidated PR #29. No commits were discarded; this branch is preserved as a recovery and reference point. Addressed review findings were replied to and resolved before consolidation.