ClusterManager: party_assignments field + set_party_assignments()

[rebelmachina]

Part of epic #112.

Depends on: nothing — standalone change in arcane-infra.

Change

All changes in crates/arcane-infra/src/cluster_manager.rs and src/lib.rs:

1. Add field to ClusterManager:

party_assignments: HashMap<Uuid, Uuid>,  // entity_id → party_id

Initialised to HashMap::new() in all constructors. Default behaviour: all party_id fields remain None — zero behaviour change for existing code.

2. Add method:

pub fn set_party_assignments(&mut self, assignments: HashMap<Uuid, Uuid>)

3. One-line change in run_evaluation_cycle() when building PlayerInfo:

// Before:
party_id: None,
// After:
party_id: self.party_assignments.get(&entity_id).copied(),

4. Export set_party_assignments is automatically available — no lib.rs change needed since ClusterManager is already re-exported.

Acceptance criteria

• All existing arcane-infra tests pass unchanged
• Test D: two entities registered under the same party UUID via set_party_assignments appear with matching party_id in the WorldStateView.players seen by the model
• Entity not in party_assignments has party_id = None in PlayerInfo
• ClusterManager::with_defaults() and ClusterManager::new() initialise party_assignments as empty (existing behaviour unchanged)
• cargo test -p arcane-infra passes
• cargo clippy -- -D warnings passes

Spec

See §4.7, §5.2 in demo_agent_group_behaviour.md.

[rebelmachina]

Implemented in PR #116. party_assignments field + set_party_assignments() added to ClusterManager; party_id populated in run_evaluation_cycle(). Test D passes — verified party_id flows into WorldStateView.players.