Cover Network backend multi-peer broadcast

[rchatham]

Summary

Adds a bounded three-peer Network backend E2E scenario to prove a secured Network-backed manager can broadcast typed messages to multiple connected peers.

Changes

• Add testNetworkBackendBroadcastsMessageToMultiplePeers.
• Start Alice, Bob, and Charlie with the same unique Bonjour service type and TLS-PSK security.
• Wait for Alice to connect to both Bob and Charlie.
• Broadcast a typed LoopbackMessage from Alice to alice.connectedPeers.
• Assert both Bob and Charlie receive the broadcast from Alice.

Verification

• swift test --filter NetworkPeerLoopbackTests — 5 tests passed
• swift test — 107 tests passed
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug — 107 tests passed
• GitHub Actions pull_request CI — Swift Package Tests and Xcode Project Tests passed
• GitHub Actions push CI — Swift Package Tests and Xcode Project Tests passed
• Correctness reviewer — no merge-blocking findings
• Security reviewer — no merge-blocking security findings

Notes / limitations

• This keeps multi-peer coverage to one bounded broadcast scenario to avoid making the real Bonjour/Network.framework test suite too broad or flaky.
• Disconnect/reconnect lifecycle E2E remains a follow-up slice.

Stack context

Stacked on PR #35 (feature/network-migration-expanded-e2e). This is the multi-peer E2E coverage slice of the Network migration stack.

[rchatham]

Superseded by consolidated PR #42. No commits were discarded; this branch is preserved as a recovery and reference point. Addressed review findings were replied to and resolved before consolidation.