feat: complete Network demo security and migration guidance

[rchatham]

Summary

Complete the adopter-facing Network.framework migration experience on top of the production/browser checkpoint:

• Add backend-specific Security controls to the expanded demo.
• Map MultipeerConnectivity to Compatible/Optional Encryption or Require Encryption.
• Map Network.framework to Unauthenticated or TLS Shared Key.
• Strictly validate Network PSKs as Base64 with at least 32 decoded bytes and fail closed without plaintext fallback.
• Generate in-memory test keys with SecRandomCopyBytes, including safe invalid-size and generator-failure handling.
• Add DemoSecurityAndInteroperabilityPlan.md and a complete app-developer migration guide.
• Add the repository-local .agents/skills/migrate-multipeer-to-network Agent Skill.
• Refresh demo documentation and screenshots.

Security behavior

• Multipeer required encryption does not authenticate individual peers because the demo uses a nil identity and accept-all certificates.
• Network TLS-PSK authenticates possession of a group secret, not individual peer identity.
• Invalid, empty, malformed, or short TLS keys block Start and never downgrade to .unauthenticated.
• Debug PSK arguments use the same validator and are unsuitable for production secret transfer.
• Key material is excluded from logs, exports, status text, accessibility values, and screenshots.
• Stop preserves the current in-memory demo key; Reset drops its references. Swift does not guarantee secure erasure.

Interoperability

MultipeerConnectivity and Network.framework are not wire-compatible. Communicating peers must currently select the same backend. A future dual-stack endpoint or gateway requires a separate architecture and threat model; this PR does not claim or implement cross-backend relaying.

Migration guidance

MultipeerConnectivityToNetworkMigrationGuide.md covers deployment targets, Local Network and Bonjour setup, backend opt-in, PSK provisioning, connection behavior, API parity, physical-device testing, observability, rollout, rollback, and mixed-version constraints. The Agent Skill turns that guidance into a repeatable implementation/review workflow.

Verification

• swift test — 175 tests passed locally.
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug — 175 tests passed locally.
• Demo iPhone 16 / iOS 18.3.1 build passed.
• Matching TLS-PSK simulator instances were previously verified to discover one another; matching/mismatched key message behavior is covered by loopback tests.
• Markdown links and Agent Skill frontmatter were validated.
• Correctness and security reviews found no merge blockers.

Screenshots

Multipeer security	Network TLS shared key

Limitations

• Network.framework remains experimental opt-in; MultipeerConnectivity remains the default.
• Multipeer required encryption and Network TLS-PSK have the trust limitations described above.
• Direct backend interoperability is unsupported.
• Physical-device security and the complete connection-behavior matrix remain release-validation requirements.

[rchatham]

GitHub prevents changing the base of a pull request that is registered as part of a stack. Closing this retained checkpoint so it can be recreated with the consolidated base; the branch, commits, review history, and this PR remain preserved.

[rchatham]

Replaced by consolidated PR #60: #60