api-handler imports the record-session fence and the fence threw
RecordSessionChangedError / SharingAccessDeniedError imported back from
api-handler — the one real import cycle in the tree, working only by
evaluation-order luck. HttpError, the SharingAuthError family and
StepUpRequiredError now live in api-errors.ts (importing nothing but
the fence contract), api-handler re-exports the identical class objects
so no importer moves, and the fence imports the leaf. A guard pins both
the identity and the leaf structure.
