Add snapshot_entities() to SpatialIndex

[rebelmachina]

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

What

Add a public method to SpatialIndex in crates/arcane-spatial/src/index.rs that exposes entity data for building WorldStateView.players:

pub fn snapshot_entities(&self) -> Vec<(Uuid, Uuid, Vec3)> {
    self.entities
        .iter()
        .map(|(entity_id, (cluster_id, position))| (*entity_id, *cluster_id, *position))
        .collect()
}

Returns (entity_id, cluster_id, position) triples for all tracked entities.

Why

SpatialIndex.entities is private. ClusterManager currently can only get cluster-level geometry via snapshot_for_view(), not individual entity data. To populate WorldStateView.players (required for AffinityEngine), ClusterManager needs entity-level access.

Acceptance criteria

• Method added to SpatialIndex
• Returns correct data for tracked entities
• cargo test -p arcane-spatial passes

Spec

See arcane-engine repo: in_08_00_integration_report.md §2.4

[rebelmachina]

Implemented in PR #77. snapshot_entities() -> Vec<(Uuid, Uuid, Vec3)> added to SpatialIndex in crates/arcane-spatial/src/index.rs. Exposes entity_id, cluster_id, position triples from the internal entities map. Used by ClusterManager to populate WorldStateView.players.