Implement AffinityEngine core — IClusteringModel::evaluate()

[rebelmachina]

Epic: #64 (AffinityEngine)
Depends on: #66 (InteractionGraph), #67 (MigrationState), #68 (AffinityConfig), #69 (ClusterScorer)

What

Implement the main AffinityEngine struct in crates/arcane-affinity/src/lib.rs:

• Implement IClusteringModel trait (evaluate(), get_model_info(), validate_view())
• Implement compute_entity_assignments() (trait extension from Extend IClusteringModel trait with compute_entity_assignments() #71)
• Interior mutability via Mutex for InteractionGraph, MigrationState, current_assignments

Seven-phase algorithm

1. Update interaction graph (decay + inject party/guild/proximity signals)
2. Tick migration cooldowns
3. Compute cluster centroids and membership from view
4. Score each entity, determine reassignments (threshold + cooldown check)
5. Spatial fallback for new entities
6. Cleanup removed entities
7. Convert per-entity assignments to merge/split ClusterDecisions

Phase 7 translation heuristics

• Merge: ≥ merge_entity_threshold entities in A want to move to B → emit Merge(A, B)
• Split: cluster has two non-interacting subgroups (connected components in interaction graph restricted to cluster) → emit Split(cluster, group_a, group_b)

Acceptance criteria

• Full evaluate() implementation with all 7 phases
• validate_view() warns on empty players
• Integration test with synthetic WorldStateView returning valid decisions

Spec

See arcane-engine repo: in_08_affinity_engine.md

[rebelmachina]

Implemented in PR #77. AffinityEngine in crates/arcane-affinity/src/lib.rs — full 7-phase IClusteringModel::evaluate() with interior mutability (Mutex). Phase 1: interaction injection (party, guild, proximity). Phase 2: compute cluster centroids/sizes. Phase 3: score each entity against all clusters. Phase 4: spatial fallback for unscored entities. Phase 5: hysteresis filter (threshold + cooldown). Phase 6: assignments to decisions (merge when ≥ merge_entity_threshold entities want to move). Phase 7: tick decay. 3 integration tests: raid group cohesion, oscillation prevention, spatial fallback.