Function engine_outdated 

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

true when the persisted graph was built by another engine generation than GRAPH_ENGINE_VERSION — older, NEWER, or before the version stamp existed (missing or unreadable meta) — and must therefore be rebuilt before its edges can be trusted. A newer stamp is what an installation finds after a rollback to an earlier release: that graph holds edges this engine’s rules did not produce and cannot interpret, so it is rebuilt, not read. Callers pair this with a node-count check: an empty graph is rebuilt regardless; a non-empty graph of another generation is rebuilt by this gate.