feat: add Network browser and production migration experience

[rchatham]

Summary

Add the adopter-facing Network browser and expanded-demo experience, then define the production setup, parity, trust, and readiness boundaries for the migration.

Changes

• Add public PeerBrowserModel for app-owned discovery and invitations.
• Integrate Network mode into the expanded demo while keeping MultipeerConnectivity selected by default.
• Add Automatic and Require Invitation behavior for both backends.
• Preserve connection modes, typed messages, logs, troubleshooting, filtering, and test checklists.
• Document why reusable framework-owned browser UI is deferred.
• Define stream/resource parity and unsupported-operation behavior.
• Define the Network peer trust roadmap and PSK limitations.
• Add Local Network privacy and Bonjour production setup guidance.
• Include the merged Network migration readiness audit.
• Incorporate all reviewed lower-checkpoint security, lifecycle, discovery, and display-name fixes.

Security and production posture

Network remains experimental opt-in. The prototype can use unauthenticated transport or TLS-PSK, but PSK proves group-secret possession rather than individual identity. Production apps must provision strong app-managed keys, treat display metadata as self-asserted, configure every Bonjour service in their own target, and complete physical-device validation.

Verification

• swift test — 163 tests passed locally at this checkpoint.
• iPhone 16 / iOS 18.3.1 Xcode tests — 163 tests passed locally.
• Expanded demo build passed locally.
• Local Network, Bonjour, browser/invitation, parity, trust, and readiness documentation reviewed holistically.

Screenshots

Multipeer / Automatic	Network / Require Invitation

Limitations

• No reusable framework-owned browser view is added; apps compose UI from PeerBrowserModel.
• Stream/resource parity remains deferred as documented.
• Network is not the default and MultipeerConnectivity is not removed.
• Direct Multipeer/Network wire interoperability is unsupported.

Stack

• Parent: feat: secure, harden, and verify the Network backend #58
• Child: feat: complete Network demo security and migration guidance #60
• Consolidates the former PRs Add peer browser model #43, Exercise Network peer discovery in the expanded demo #44, Defer reusable Network browser UI #45, Define Network stream and resource parity boundary #46, and Define Network peer trust roadmap #47, plus readiness audit PR Add Network migration readiness audit #50 already merged into this branch. All branches remain preserved.

Stack update

• Merged the updated network-config base that includes the actor-backed Observable bridge from PR refactor: unify observables behind actor bridge #63.
• Preserves the browser/demo/local-network guidance changes and updates browser-model tests to drive async observable events deterministically.

Verification

• swift test passed after merging the updated base.
• swift build passed after merging the updated base.