Add peer security configuration

[rchatham]

Summary

• introduce PeerSecurityConfiguration, PeerCertificatePolicy, and PeerInvitationPolicy
• thread session security settings into MCSession creation
• replace hardcoded certificate acceptance with explicit policy handling
• preserve .receivedCertificate as an observation-only compatibility event
• add PeerDiscoveryInfo support for advertised Bonjour discovery metadata
• surface browser-provided discovery metadata with foundPeerWithDiscoveryInfo while preserving existing foundPeer events
• replace blind .automatic invitation acceptance with policy-based handling
• preserve .receivedInvitation for manual invitation policy and non-automatic connection types
• add non-breaking validation helpers for MultipeerConnectivity service types and display names
• document display name privacy and service type constraints
• expose PeerConnectivityUI browser peer filtering via discovery metadata
• fix the demo workspace local package reference so it resolves from repo/worktree checkouts
• retain the PeerConnectivityUI browser delegate without subclassing MCBrowserViewController
• ignore local SwiftPM/Xcode workspace state under .swiftpm/
• tighten service type validation to reject additional values that MultipeerConnectivity rejects at initialization
• preserve automatic invitation observation by emitting compatibility .receivedInvitation events while keeping policy decisions authoritative
• document event compatibility notes for discovery metadata and automatic invitation policy events
• merge the latest master demo/test improvements and log discovery metadata events in the expanded demo app
• add changelog notes for the peer security configuration release and source compatibility considerations

Verification

• git diff --check
• swift build && swift test
• swift test after Phase 2 discovery metadata changes
• swift test after Phase 3 invitation policy changes
• swift test after Phase 4 validation/privacy hardening changes
• swift test after Phase 5 browser filtering changes
• swift test after certificate compatibility fix
• swift test after demo coverage changes
• xcodebuild build -workspace PeerConnectivity.xcworkspace -scheme PeerConnectivityUI -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug
• xcodebuild test -workspace PeerConnectivity.xcworkspace -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug
• xcodebuild build -project PeerConnectivityDemo.xcodeproj -scheme PeerConnectivityDemo -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug
• git diff --check, swift test, and demo xcodebuild build after filtered-browser stabilization
• git diff --check, swift test, and xcodebuild test -workspace PeerConnectivity.xcworkspace -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug after review-fix updates
• git diff --check and swift test after changelog/demo documentation updates
• final pre-merge verification after merging latest master: git diff --check, swift build, swift test, xcodebuild build for PeerConnectivityUI, xcodebuild test for PeerConnectivity, demo xcodebuild build, and two-simulator generated XCTest smoke test all passed

Notes

• Security defaults remain backward-compatible: optional encryption, no security identity, accept-all certificate policy, and accept-all automatic invitation policy.
• Certificate decisions now live in PeerSecurityConfiguration.certificatePolicy.
• .receivedCertificate remains observable for API compatibility, but its supplied handler is observation-only and does not affect the certificate decision.
• Discovery info is public, unauthenticated Bonjour TXT record metadata; docs warn not to include secrets, tokens, emails, stable user IDs, or sensitive device information.
• Existing .foundPeer(peer:) events are preserved; metadata is exposed through the additional .foundPeerWithDiscoveryInfo(peer:discoveryInfo:) event.
• Source compatibility note: PeerConnectionEvent has a new .foundPeerWithDiscoveryInfo(peer:discoveryInfo:) case. Existing exhaustive switches over PeerConnectionEvent may need a new case or default.
• Invitation context is received before session establishment and should be treated as public, unauthenticated metadata.
• In automatic mode, non-manual invitation policies still emit .receivedInvitation for observation/API compatibility; the emitted handler is a no-op and policy decisions remain authoritative.
• Service type and display name validation helpers are advisory/non-breaking; initialization behavior is unchanged. Service type validation now rejects leading/trailing hyphens, consecutive hyphens, and all-numeric names to better match MultipeerConnectivity constraints.
• Browser peer filtering is synchronous and defaults to presenting all peers when no filter is supplied.
• The demo app can be run on two simulators/devices to exercise advertising, browsing, connection state, typed messages, raw data, resources, and event logging. Discovery metadata events are logged as peer.found.metadata.

Follow-up

• Potential future work: typed Codable helpers for invitation context or stronger service type wrapper APIs.