Add opt-in Network backend selector

[rchatham]

Summary

Adds an explicit opt-in backend selector for the Network.framework migration while preserving MultipeerConnectivity as the default runtime backend.

Changes

• Add public PeerConnectionBackend:
  • .multipeerConnectivity remains the default.
  • .networkFramework is explicitly opt-in and documented as incomplete parity.
• Add public PeerConnectionManager.backend for debugging/tests.
• Extend the existing public initializer with a defaulted backend: parameter, preserving existing call sites.
• Route .networkFramework through the internal Network transport factory when OS availability permits.
• Fail explicitly instead of silently falling back on OS versions where Network.framework backend is unavailable.
• Document multipeerSession as MC-only.
• Add test coverage proving:
  • default initializer remains MC-backed
  • explicit MC backend remains MC-backed
  • Network backend constructs Network transports
  • Network .automatic starts without switching default behavior
  • Network .inviteOnly starts without MC advertiser assistant UI
  • internal transport-factory initializer remains available and unaffected

Limitations / non-goals

• No default backend change.
• No local simulator Alice/Bob loopback test yet.
• No SwiftUI browser replacement yet.
• No Network stream/resource parity yet.
• No public trust/security guarantee for the experimental Network path yet.

Verification

• swift test — 94 tests passed
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug — 94 tests passed
• GitHub Actions Swift Package Tests — passed
• GitHub Actions Xcode Project Tests — passed
• Reviewer pass — no merge-blocking findings
• Security reviewer pass — no merge-blocking security findings

Stack context

Stacked on PR #28 (feature/network-migration-discovery-metadata). This is the opt-in backend selector slice before local simulator loopback integration.