Secure Network backend transport with PSK

[rchatham]

Summary

Adds an authenticated encrypted option for the experimental Network framework backend using TLS pre-shared keys, and updates loopback coverage to exercise the secured path.

Changes

• Add public PeerConnectionNetworkSecurity:
  • .unauthenticated preserves source-compatible plaintext diagnostics/migration behavior.
  • .preSharedKey(Data) configures TLS-PSK for Network framework connections.
• Add networkSecurity: to PeerConnectionManager with default .unauthenticated.
• Thread Network security configuration through the Network transport factory, session transport, browser, listener, and outbound connections.
• Configure NWParameters with TLS options and sec_protocol_options_add_pre_shared_key when .preSharedKey is selected.
• Update README and API docs with the secured opt-in example and plaintext caveat.
• Update loopback E2E test to use TLS-PSK.
• Add negative coverage that mismatched PSKs do not connect.
• Add coordinator negative tests for unsupported handshake versions and duplicate identity spoof attempts.

Verification

• swift test --filter NetworkPeerCoordinatorTests — passed
• swift test --filter NetworkPeerLoopbackTests — passed
• swift test — 98 tests passed
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug — 98 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 after rerunning an initial Xcode job failure
• Correctness reviewer — no remaining merge-blocking findings; matched-key loopback passed locally, resolving the only functionality concern raised
• Security reviewer — no merge-blocking security findings

Notes / limitations

• .networkFramework remains opt-in and still lacks streams/resource/UI parity.
• .unauthenticated remains available for source compatibility and diagnostics, but docs explicitly mark it as plaintext and not for sensitive data.
• This PR addresses the first security hardening slice. Follow-ups should add connection hardening: handshake timeout, connection caps, backoff/retry, and stricter cleanup behavior.

Stack context

Stacked on PR #31 (feature/network-migration-local-loopback). This is the secure transport hardening 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Superseded by consolidated PR #42.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.