Extend IClusteringModel trait with compute_entity_assignments()

[rebelmachina]

Epic: #64 (AffinityEngine)
No dependencies — can be done independently

What

Add a per-entity assignment method to IClusteringModel in crates/arcane-core/src/clustering_model.rs:

fn compute_entity_assignments(
    &self,
    view: &WorldStateView,
) -> std::collections::HashMap<Uuid, Uuid> {
    std::collections::HashMap::new()
}

Key design decisions

• Default implementation returns empty map — RulesEngine compiles without changes
• &self not &mut self — matches existing trait constraint
• Entities not in the map retain current assignment (diff semantic)
• Exists for testing, benchmarking, and future ClusterManager evolution

Changes

• crates/arcane-core/src/clustering_model.rs — add method to trait (line ~78)
• No changes to arcane-rules (default impl covers it)
• No changes to arcane-infra (ClusterManager continues calling evaluate())

Acceptance criteria

• Method added with default implementation
• cargo build --workspace passes
• cargo test --workspace passes (existing tests unaffected)

Spec

See arcane-engine repo: in_08_05_trait_extension.md

[rebelmachina]

Implemented in PR #77. Default method compute_entity_assignments(&self, view) -> HashMap<Uuid, Uuid> added to IClusteringModel trait in crates/arcane-core/src/clustering_model.rs. Default returns empty map; AffinityEngine overrides it. Backward-compatible — existing implementors (RulesEngine) get the default for free.