feat(core): peer scoring service and MemberInfo UI type

[seemenkina]

Summary

• Add peer scoring infrastructure split across core (protocol vocabulary) and app (service + defaults), following the core/app layer separation
• Introduce MemberInfo struct in UI protocol to carry member scores alongside addresses
• Storage is group-scoped: PeerScoreStorage keyed by (group_id, member_id), service designed as one-per-User
• apply_consensus_result now returns ConsensusApplyResult with Vec

What's where

• core::peer_scoring — ScoreEvent (9 variants), ScoreOp, ScoringProvider trait, PeerScoreStorage trait, ScoringConfig, ConsensusApplyResult
• app::peer_scoring — PeerScoringService<S, P>, InMemoryPeerScoreStorage, FixedScoringProvider
• de_mls_ui_protocol — MemberInfo { address, score } replaces String in GroupMembers event
• Gateway — maps member addresses to MemberInfo with default score 100

Test plan

• New peer scoring tests (add/remove, apply events, threshold detection, saturation, cross-group isolation, determinism)
• All existing tests pass
• Clippy clean, docs build without warnings
• Full workspace builds (de-mls, ui-protocol, gateway, desktop-ui)