test: add comprehensive unit tests for ring module

- Fix Score::Ord to use total_cmp() for consistent NaN handling
  (previous implementation would treat NaN as equal to everything)
- Add score.rs to mod.rs exports
- Add ~100 new tests covering:
  - score.rs: ordering, NaN/infinity handling, sorting
  - location.rs: distance calculations, IPv4/IPv6 mapping, hashing
  - peer_key_location.rs: PeerAddr variants, ordering, serialization
  - connection_manager.rs: should_accept logic, transient connections,
    routing, peer identity updates

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: sanity