Identity fallbacks should preserve evidence authority and provenance

When an external check requires identifier A but accepted evidence may contain only identifier B, keep the fallback inside the existing observation boundary: prefer A when directly observed, resolve B→A through one strict provider adapter, validate the returned identity against B, and persist which source produced A. Provider absence or malformed data should remain unavailable rather than becoming a negative business result. Deduplicate lookups and cap concurrency when no batch endpoint exists. This is useful for informational checks; it does not justify substituting booked, expected, or otherwise less-authoritative identity.

Applies when: a provider needs one identifier but the authoritative evidence sometimes supplies a safely resolvable alternate identifier. Keywords: identifier fallback source authority provenance provider validation unavailable bounded concurrency