feat: secure, harden, and verify the Network backend

[rchatham]

Summary

Secure, harden, and verify the opt-in Network.framework backend across loopback, multi-peer, reconnect, cleanup, and API-contract scenarios.

Changes

• Add local Network loopback verification.
• Add TLS pre-shared-key transport while retaining explicitly unauthenticated diagnostic mode.
• Harden connection lifecycle, duplicate replacement, reconnect handling, timeout/cap policy, cancellation, and cleanup.
• Define backend API expectations and unsupported-operation behavior.
• Add expanded message, multi-peer broadcast, reconnect, and cleanup coverage.
• Isolate Network loopback CI strategy and defer runtime policy configuration deliberately.
• Document backend setup and migration behavior.
• Guarantee loopback teardown on every test path.
• Sanitize supplied and host-derived display names to non-empty values within the 63-byte UTF-8 Multipeer limit for both backends.
• Incorporate the reviewed adapter, discovery, identity, and frame-decoder fixes from the updated parent.

Security

Network TLS-PSK authenticates possession of a group secret, not an individual peer identity. Unauthenticated transport remains explicit and is not suitable for sensitive production traffic. Display names and Bonjour identifiers remain unsuitable as authorization principals.

Verification

• swift test — 159 tests passed locally at this checkpoint.
• iPhone 16 / iOS 18.3.1 Xcode tests — 159 tests passed locally.
• Demo build passed locally.
• Coverage includes matching/mismatched PSKs, lifecycle state transitions, loopback messaging, multi-peer broadcast, reconnect, cleanup, display-name boundaries, and unsupported APIs.
• Correctness and security reviews found no blockers.

Limitations

Network remains experimental opt-in. Stream/resource parity, reusable browser UI, production trust, and adopter UX are handled in later checkpoints.

Stack

• Parent: feat: add Network migration infrastructure and opt-in backend #57
• Child: feat: add Network browser and production migration experience #59
• Consolidates the former PRs Verify Network backend local loopback #31, Secure Network backend transport with PSK #32, Harden Network backend connection lifecycle #33, Define Network backend API expectations #34, Expand Network backend E2E coverage #35, Cover Network backend multi-peer broadcast #36, Handle Network backend reconnects #38, Document Network backend migration guide #39, Cancel removed Network connections #40, and Isolate Network loopback CI #41; their commits and branches remain preserved.