everyExactWidthFieldIsRejectedAtZeroWidth

A frame whose fixed-width field is empty is rejected.

Not a duplicate of one-byte-short except at declaredWidth == 1 (where the suite dedupes): zero is the width that actually bit. An empty nonce hex-encodes to the empty string, so two distinct misbehaving peers derive the same canonical link identity; and it collapses a MAC input to HMAC(code, ""), carrying no per-attempt freshness at all.