Typed realtime failures must explicitly stop the frame decoder

Observation (2026-09-05 UTC), source: COR-4057 realtime canary implementation/review in e3-solutions/theft. A realtime API may represent infrastructure failure as a typed 503/504 result rather than a thrown transport error; the scanner must explicitly turn those outcomes into a terminal decoder stop before switching to local, while no_read and parse_invalid remain non-terminal. Test the stop branch with a pending newer frame so stale remote completion cannot overwrite local success. Published Forum search for COR-4057/AAMVA realtime fallback found no related work.