Add core and hook mapper regression coverage for the thread-origin contract behind #83302.\n\nThe tests prove a flat reply target can coexist with a thread-addressable OriginatingTo, and hook canonical conversation mapping keeps following OriginatingTo.\n\nProof: focused Vitest, autoreview, Testbox check:changed tbx_01krwaztbwm13sx9e4sbyyz4c1, and CI run 26008670388 passed.