Function entity_content_hash 

pub fn entity_content_hash<T: Serialize>(entity: &T) -> Result<u64, String>

Per-entity content hash: the entity’s normalized YAML serialization (stable field order, defaulted-empty fields omitted — the same normalization gateway_to_yaml applies to a whole Gateway) through the same DefaultHasher as the snapshot content hash. Two entities of one kind hash equal exactly when their normalized serializations are equal, so source key order and omitted defaults do not affect the result. Same caveats as SnapshotInfo::content_hash: a per-process change-detection token, not stable across Rust versions, not a content digest.