Remark: What the equivalence does not supply

Theorem thm:jetextension constructs an extension from already supplied coefficient extensions. It does not prove their existence, a scaling estimate, a contact normalization, or an admissible graph wavefront bound. For $m=3$, it provides the elementary supported-jet interface relevant to the supplied XIV-J comparison XIVj (Theorem thm:conditionalextension, lines 1427–1470).