test: add comprehensive unit tests for ring module

[sanity]

Problem

The ring module had low test coverage in several key files:

• connection_manager.rs: 22.1%
• peer_key_location.rs: 18.0%
• location.rs: 64.0%
• score.rs: 0% (not even included in mod.rs)

Additionally, the Score type's Ord implementation had a potential bug where partial_cmp().unwrap_or(Equal) would treat NaN values as equal to everything, violating Ord requirements.

Solution

Bug Fix

• Fixed Score::Ord to use total_cmp() for consistent NaN handling. The previous implementation would cause undefined behavior in sorting algorithms when NaN values were present.

New Tests (~100 tests added)

score.rs (8 tests):

• Normal value ordering, negative values, NaN handling, infinity handling
• Sorting with mixed values (verifies NaN sorts consistently)

location.rs (~35 tests):

• Distance calculations (wrapping at 0.5, normalization)
• Location arithmetic (add distance, boundary conditions)
• IPv4/IPv6 address mapping (Sybil mitigation via subnet masking)
• Hash consistency for HashMap/HashSet usage
• Contract key location computation

peer_key_location.rs (~25 tests):

• PeerAddr variants (Unknown/Known)
• Ordering (unknown addresses sort last)
• Display formatting
• IPv6 support
• Serialization

connection_manager.rs (~30 tests):

• Initial state and configuration (gateway vs regular mode)
• should_accept logic (min/max connections, already connected peers)
• Transient connection budget enforcement
• Routing with skip lists
• Peer identity updates

Test Plan

• All 148 ring module tests pass
• Clippy passes with no warnings
• Code formatted

[AI-assisted - Claude]