Cancel removed Network connections

[rchatham]

Summary

Makes Network connection teardown explicit when receive loops end or coordinator state removes a connection.

Changes

• Cancel NWConnection when the receive loop sees an error or EOF before emitting the synthesized .cancelled state.
• Cancel pending or registered coordinator connections when removeConnection(_:) removes them from coordinator state.
• Keep cancellation outside the coordinator serial queue so cancellation-driven callbacks can safely re-enter cleanup without deadlocking.
• Add coordinator coverage proving pending removals cancel without emitting a session event.
• Extend registered removal coverage to verify the removed connection is cancelled.

Verification

• swift test --filter NetworkPeerCoordinatorTests — 18 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
• 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

• This keeps cleanup idempotent; duplicate .cancelled callbacks from Network.framework remain safe no-ops after the first removal.
• No public API changes.

Stack context

Stacked on PR #39 (feature/network-migration-docs-guide). This is the explicit Network connection cleanup slice of the migration stack.

[Copilot]

Pull request overview

This PR tightens Network.framework connection teardown semantics so NWConnection instances are explicitly cancelled when a receive loop terminates (EOF/error) and when the coordinator removes a pending or registered connection, while avoiding coordinator-queue deadlocks from cancellation-driven callbacks.

Changes:

• Cancel NWConnection when the receive loop hits EOF/error before synthesizing a .cancelled state.
• Update NetworkPeerCoordinator.removeConnection(_:) to cancel pending/registered connections after removing them from coordinator state (and outside the coordinator serial queue).
• Extend coordinator unit tests to assert removed connections are cancelled and that pending removals do not emit session events.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Sources/NetworkPeerTransport.swift	Cancels the underlying NWConnection when the receive loop ends due to EOF/error before emitting the synthesized .cancelled state.
Sources/NetworkPeerCoordinator.swift	Ensures removing a connection also cancels it, with cancellation occurring outside the coordinator queue to avoid deadlock risk.
PeerConnectivityTests/NetworkPeerCoordinatorTests.swift	Adds/extends coverage verifying cancellations for both registered and pending connection removals (and no session event for pending removal).

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[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.