Isolate Network loopback CI

[rchatham]

Summary

Isolates real Network.framework/Bonjour loopback tests into focused retryable CI steps while keeping unit test failures easier to diagnose.

Changes

• Keep regular Swift Package and Xcode test jobs for the full suite.
• Skip NetworkPeerLoopbackTests from the main CI passes:
  • SwiftPM uses an XCTest skip guard when CI=true and PEERCONNECTIVITY_RUN_NETWORK_E2E is not set.
  • Xcode uses explicit -skip-testing:PeerConnectivityTests/NetworkPeerLoopbackTests in the main simulator test step.
• Add focused retryable Network loopback steps:
  • swift test --filter NetworkPeerLoopbackTests
  • xcodebuild test ... -only-testing:PeerConnectivityTests/NetworkPeerLoopbackTests
• Document the CI split in NetworkBackendGuide.md.

Verification

• CI=true swift test --filter NetworkPeerLoopbackTests — 6 tests skipped as expected
• PEERCONNECTIVITY_RUN_NETWORK_E2E=1 CI=true swift test --filter NetworkPeerLoopbackTests — 6 tests passed
• swift test — 109 tests passed
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug — 109 tests passed
• CI=true xcodebuild test ... -skip-testing:PeerConnectivityTests/NetworkPeerLoopbackTests — 103 tests passed
• PEERCONNECTIVITY_RUN_NETWORK_E2E=1 CI=true xcodebuild test ... -only-testing:PeerConnectivityTests/NetworkPeerLoopbackTests — 6 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

• Focused Network loopback failures still fail CI after one retry; this does not make E2E optional.
• Local swift test / xcodebuild test continue running the Network loopback tests unless the caller explicitly uses CI-style skip/filter arguments.

Stack context

Stacked on PR #40 (feature/network-migration-connection-cleanup). This is the CI E2E flake-isolation 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Superseded by consolidated PR #42.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.