Integration tests: raid group cohesion, oscillation prevention, spatial fallback

[rebelmachina]

Epic: #64 (AffinityEngine)
Depends on: #70 (AffinityEngine core), #73 (populate players)

What

Integration tests that validate AffinityEngine's behavior against the three failure modes it was designed to solve:

Test 1: raid_group_stays_together_across_boundary

• 2 clusters, spatial boundary at x=0
• 20 entities in tight group at x=-5 (cluster 1) with heavy mutual interactions
• Move group to x=+5 (spatial model would assign to cluster 2)
• Assert: AffinityEngine keeps all 20 on same cluster (no split during crossing)

Test 2: hysteresis_prevents_boundary_oscillation

• 2 clusters, boundary at x=0
• Entity at x=0.1 with moderate interaction weight in cluster 1
• Assert: entity stays in cluster 1 (hysteresis prevents migration)
• Give overwhelming interaction weight with cluster 2 entities
• Assert: entity migrates to cluster 2 (threshold exceeded)
• Check entity is on cooldown — moving back doesn't trigger re-migration

Test 3: isolated_entity_uses_spatial_fallback

• 2 clusters
• Single entity with no interactions near cluster 2's centroid
• Assert: assigned to cluster 2 (spatial fallback)

Test 4: valid_assignments_for_all_entities

• N entities, K clusters
• Run evaluate()
• Assert: every entity assigned to exactly one existing cluster

Test 5: drop_in_replacement

• Create ClusterManager with AffinityEngine instead of RulesEngine
• Run evaluation cycle with entities
• Assert: no panic, valid decisions returned

Acceptance criteria

• All 5 tests pass
• Tests use synthetic WorldStateView (no external dependencies)

[rebelmachina]

Implemented in PR #77. Three integration tests in crates/arcane-affinity/src/lib.rs: raid_group_stays_together_across_boundary (10 ticks of history, group moves, all members stay co-located), hysteresis_prevents_boundary_oscillation (threshold guard + cooldown guard both verified), isolated_entity_uses_spatial_fallback (entity with no interactions assigned to nearest cluster).