everyExactWidthFieldsRigVariesTheWidthAndNothingElse

The rig varies the width and nothing else: its frames grow strictly with the declared width, so a rig returning one constant frame, or unrelated frames, is caught.

Deliberately not the stronger linear form (size(w) == base + w), true though it is for every in-tree field today. A length-delimited encoding widens its own length header as the field crosses a size boundary — CBOR's byte-string header grows from one byte to two at 24 — so the linear check would false-red a perfectly correct rig for a 24-byte field. Strict monotonicity holds for every encoding. Do not "strengthen" this.