Function supported_version 

pub fn supported_version(migrations: &[Migration]) -> u32

The top synced-schema version this binary supports: the count of registered migrations. run_migrations validates the set is 1..=N contiguous, so the count is the highest version (and 0 for an empty ladder — no synced schema). The snapshot bootstrap gate compares an incoming snapshot’s version against this before adopting the image, so it lives beside the ladder rather than being re-derived at each bootstrap call site.