The persistence seam translates the format edge's
SessionFormatUnsupportedMigrationError into SessionFormatUnsupportedError
(with the "source vN artifact remains unchanged" suffix) before the error
escapes, so the exact-name check never matched on any supported dsh
baseline: the skip summary never fired and every refused legacy session
kept its own WARN (issue #75). Classify by the SessionFormatUnsupported
prefix instead, and cover both faces of the translation in the fixture.