Migrate toward Network framework transport

[rchatham]

Summary

Begins migrating PeerConnectivity away from direct MultipeerConnectivity internals by adding framework-neutral seams and a Network framework prototype path while preserving the existing public API.

Changes

• Add a Network framework migration plan documenting dependency surfaces, risks, phases, and testing strategy.
• Add internal transport seams around session, browser, advertiser, and advertiser assistant responsibilities.
• Introduce framework-neutral PeerIdentity while keeping Peer source-compatible with current MultipeerConnectivity-backed behavior.
• Add Network protocol primitives for Bonjour service naming, handshakes, and framed messages.
• Add a stream-safe Network frame decoder for partial/coalesced TCP receives while preserving frame kind.
• Add a prototype Network transport using NWConnection, NWListener, and NWBrowser with peer-to-peer enabled.
• Add duplicate connection resolution and connection state mapping for Network-backed peers.
• Add Network data sending over .data frames.
• Add a Network peer coordinator that bridges handshakes, data frames, connection registration, and discovery events into existing PeerConnectivity event seams.
• Reject self discovery and self handshakes in the Network coordinator.

Verification

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

Notes / Follow-ups

• Network backend is not publicly selectable yet; current public behavior remains MultipeerConnectivity-backed.
• multipeerSession: MCSession remains the largest public API compatibility constraint for any future iOS 27+ MC removal path.
• Resource transfer, stream compatibility, invite UI semantics, and a public backend selector remain follow-up work.
• The iOS 27 MultipeerConnectivity deprecation/removal assumption still needs verification against Apple SDK headers or release notes.