Define Network backend API expectations

[rchatham]

Summary

Defines the initial API expectations for the experimental Network framework backend and makes the browser UI path fail predictably instead of reaching MultipeerConnectivity-only state.

Changes

• Document Network backend support in public API comments:
  • .automatic discovery/connect is supported.
  • .custom + invitePeer is supported for app-owned peer selection.
  • .inviteOnly does not provide MultipeerConnectivity UI with .networkFramework.
  • sendData and sendMessage are supported.
  • streams/resources remain unsupported and report explicit errors.
  • multipeerSession remains MultipeerConnectivity-only.
• Add a README support matrix comparing MultipeerConnectivity and Network framework behavior.
• Document app-owned Network peer selection using .foundPeer / .lostPeer and invitePeer, recommending .preSharedKey for authenticated encrypted sessions.
• Clarify that networkSecurity applies only to .networkFramework; MultipeerConnectivity retains its own MCSession security behavior.
• Update PeerConnectivityUI.browserViewController to return nil for Network-backed managers instead of touching multipeerSession and triggering a programmer-error fatal path.
• Add tests covering networkSecurity storage/default behavior and the fact that MultipeerConnectivity ignores the Network security configuration.

Verification

• swift test — 104 tests passed
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug — 104 tests passed
• Correctness reviewer — no merge-blocking findings
• Security reviewer — no merge-blocking security findings

Notes / limitations

• This PR intentionally keeps the Network support matrix as documentation plus predictable UI behavior, not a broader public capability/configuration API.
• Streams/resources remain follow-up work.
• A SwiftUI/UIKit-neutral browser replacement remains the next UI-oriented migration slice.

Stack context

Stacked on PR #33 (feature/network-migration-connection-hardening). This is the API expectation stabilization 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Superseded by consolidated PR #42.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.