Readonly~driverReadonly~schemaReadonlyconfigTransactional read-modify-write. Reads the latest persisted state, runs
fn to mutate it, validates, and persists — atomically across processes
when the driver supports it (preventing lost updates from concurrent
writers). Falls back to load→mutate→save when it does not.
fn may mutate the draft in place or return a replacement.
Proxy to access configuration properties directly. Example: kfg.config.database.port