Harden Network backend connection lifecycle

[rchatham]

Summary

Adds the next Network backend hardening slice: bounded pending/connected connection lifecycle, handshake timeout cleanup, and transport state cleanup hooks.

Changes

• Add internal NetworkPeerConnectionPolicy with defaults:
  • handshakeTimeout = 10s
  • maxPendingConnections = 16
  • maxConnectedPeers = 8
• Cancel pending connections that do not complete the PeerConnectivity handshake before timeout.
• Enforce pending connection and connected peer caps before accepting more Network connections.
• Re-check connected peer capacity when handshakes arrive, so concurrently pending connections cannot overfill the registry.
• Cancel and cleanup pending timeout work items when connections register, fail validation, are removed, or all connections are stopped.
• Add NetworkPeerConnection.setStateHandler and wire Network .failed / .cancelled states back into coordinator cleanup.
• Add focused coordinator coverage for:
  • handshake timeout cancellation
  • pending connection cap
  • connected peer cap before new pending connection
  • connected peer cap when pending handshakes complete concurrently

Verification

• swift test --filter NetworkPeerCoordinatorTests — passed
• swift test --filter NetworkPeerLoopbackTests — passed
• swift test — 102 tests passed
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug — 102 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

• Policy is internal for now to keep this PR focused and avoid public API churn before the Network backend is promoted.
• Backoff/retry tuning and public configurability are deferred until we know the desired production defaults.
• .networkFramework remains opt-in and still lacks streams/resource/UI parity.

Stack context

Stacked on PR #32 (feature/network-migration-secure-transport). This is the connection lifecycle 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Superseded by consolidated PR #42.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.