Add Network discovery metadata

[rchatham]

Summary

Adds Bonjour discovery identity metadata for the Network transport stack so discovered Network browser results can map to framework-neutral PeerIdentity values and endpoints.

Changes

• Add PeerNetworkDiscoveryInfo for TXT record encode/decode.
• Validate discovery metadata:
  • required identifier/display name/protocol version
  • supported protocol version only
  • non-empty values
  • bounded TXT-record-safe advertised lengths
• Advertise local peer discovery metadata from NetworkPeerListener.
• Parse NWBrowser.Result Bonjour metadata in NetworkPeerBrowserTransport.
• Store PeerIdentity -> NWEndpoint mappings for later invitePeer connection attempts.
• Ignore self-discovery in the browser adapter.
• Keep malformed/missing metadata ignored.

Non-goals

• No public backend selector yet.
• No default backend change.
• No trust/auth claim for TXT metadata; it is discovery routing data only.
• No local simulator loopback integration yet.

Verification

• swift test — 88 tests passed
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug — 88 tests passed

Stack context

Stacked on PR #27 (feature/network-migration-transport-adapters). This is the discovery metadata slice in the Network migration stack.