everyFixedWidthHeaderAcceptsAFrameAtOrOneByteOverTheHeader

A frame of exactly the header width is accepted, and so is one a byte longer.

The long arm is the property that makes a header not an ExactWidthField: the extra byte is payload, and refusing it would be the bug. Asserting it here is what stops a later reader "completing" the symmetry by demanding a rejection on the long side.