Unit tests for all arcane-affinity modules

[rebelmachina]

Epic: #64 (AffinityEngine)
Depends on: #66, #67, #68, #69, #70 (all module implementations)

What

Comprehensive unit tests for each module in arcane-affinity:

interaction_graph.rs

• record_interaction creates entry / adds to existing (not replaces)
• EntityPair canonical ordering — (A,B) and (B,A) produce same key
• tick() applies decay correctly
• tick() GCs entries below threshold at correct interval
• neighbors() returns all partners with correct weights
• remove_entity() cleans up all pairs involving that entity
• pair_count() accurate after insertions, removals, GC

hysteresis.rs

• New MigrationState has no cooldowns
• record_migration sets cooldown correctly
• is_on_cooldown returns true during period
• tick() decrements, entries removed at 0
• Cooldown expires after exactly cooldown_ticks ticks
• remove_entity clears cooldown

scorer.rs

• Entity with all interaction partners in cluster A scores highest for A
• Entity with no interactions falls back to spatial proximity
• Capacity penalty reduces score for full clusters
• Capacity penalty doesn't zero the score (allows overflow)
• spatial_weight=0.0 → pure interaction scoring
• spatial_weight=1.0 → pure spatial scoring

lib.rs (AffinityEngine)

• evaluate() returns valid assignments for all entities
• validate_view() warns on empty players
• get_model_info() returns correct metadata

Acceptance criteria

• cargo test -p arcane-affinity passes with all tests
• No tests depend on external state (Redis, SpacetimeDB, etc.)

[rebelmachina]

Implemented in PR #77. Unit tests across all arcane-affinity modules: 7 in interaction_graph.rs, 5 in hysteresis.rs, 4 in scorer.rs, plus integration tests in lib.rs. 25 tests total, all passing. Covered: decay, GC, canonical pair ordering, self-interaction guard, capacity penalty floor, spatial fallback, cooldown expiry.