Function engine_outdated 

pub fn engine_outdated(project_root: &str) -> bool

true when the persisted graph was built by an engine older than GRAPH_ENGINE_VERSION — or predates the version stamp entirely (missing or unreadable meta) — and must therefore be rebuilt before its edges can be trusted. Callers pair this with a node-count check: an empty graph is rebuilt regardless; a non-empty-but-outdated graph is rebuilt by this gate.