Function migrate_definitions_global_once 

pub fn migrate_definitions_global_once(
    home: &Path,
    store: &DefinitionStore,
) -> Result<DefMigrateStats, DefStoreError>

Scan every channel AND dev-branch objects.db under home for user agents and backfill them into the global definition store. Re-runs once per MIGRATION_VERSION bump (marker in store.root()); read-only on every scanned SQLite, and tolerant of older schemas (missing columns).