Serialize event observer mutations

[rchatham]

Summary

Continues the Network migration PR series by making the existing event observer pipeline safe for concurrent delegate and Network callback delivery.

Changes

• Protect Observable observer/value storage with NSLock.
• Protect MultiObservable observer/value storage with NSLock.
• Snapshot observer lists under lock and invoke callbacks outside the lock to avoid reentrant deadlocks.
• Add removeAllObservers() helpers instead of directly mutating observer storage from callers.
• Protect PeerConnectionResponder listener bookkeeping with NSLock.
• Update PeerConnectionManager.stop() to clear observers through the new synchronized helper.
• Add concurrency-focused tests for concurrent observer mutation, event delivery, self-removal during delivery, and responder listener removal.

Verification

• swift test
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug -enableThreadSanitizer YES -only-testing:PeerConnectivityTests/ObservableThreadSafetyTests

Notes

• This is intended as a stacked follow-up on PR Add Network migration CI baseline #23 (feature/network-migration-ci-baseline).
• No public API or runtime backend selection changes are included.
• Callback delivery ordering is unchanged for single-threaded callers; concurrent producers are serialized only around observer storage, not globally ordered across producer queues.

[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Superseded by consolidated PR #29.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.