Add arcane-affinity as optional dependency in arcane-infra with model selection

[rebelmachina]

Epic: #64 (AffinityEngine)
Depends on: #70 (AffinityEngine core)

What

1. Add arcane-affinity as an optional dependency in crates/arcane-infra/Cargo.toml under a feature flag:

arcane-affinity = { path = "../arcane-affinity", optional = true }

[features]
affinity-clustering = ["dep:arcane-affinity"]

2. Add a model selection constructor to ClusterManager:

pub fn with_model(model_type: &str) -> Self {
    let model: Arc<dyn IClusteringModel> = match model_type {
        #[cfg(feature = "affinity-clustering")]
        "affinity" => Arc::new(arcane_affinity::AffinityEngine::default()),
        _ => Arc::new(RulesEngine::new()),
    };
    Self::new(model, Arc::new(LocalPool::default()), SpatialIndex::new())
}

Acceptance criteria

• cargo build --workspace passes (default features, no affinity)
• cargo build --workspace --features arcane-infra/affinity-clustering passes
• ClusterManager::with_model("affinity") creates an AffinityEngine instance
• ClusterManager::with_model("rules") creates a RulesEngine instance (default)

Spec

See arcane-engine repo: in_08_00_integration_report.md §2.6

[rebelmachina]

Implemented in PR #77. arcane-affinity added as optional dep in arcane-infra/Cargo.toml under affinity-clustering feature. ClusterManager::with_model(model_type: &str) constructor added — passes "affinity" to get AffinityEngine::default(), guarded by #[cfg(feature = "affinity-clustering")]. All other model strings fall back to RulesEngine.