Normalizes MaxMind GeoLite2-City / GeoIP2-City result maps into the canonical
GoodAnalytics.Geo.Normalizer.normalized shape.
Locus emits binary keys in MMDB results. We tolerate atom keys as well (for
hand-built test fixtures), but only via the explicit @known_keys set —
never via String.to_atom/1, to avoid atom-table exhaustion on adversarial
input.