FixedWidthHeader
A fixed-width header followed by an arbitrary payload — the other shape a documented width takes on a wire, and deliberately not an ExactWidthField.
The distinction is the whole reason this is a separate declaration rather than a third arm of the exact-width one. For a header, a frame one byte longer than the header is a frame with a one-byte payload: it must be accepted, and a codec that refused it would be broken. Only the short side is a width violation. Folding the two shapes together would either impose a false obligation on every header or quietly relax the exact-width contract to "short is enough" — and that relaxation is exactly how a fixed-width field stops being checked on its long side.
Properties
builds a frame of exactly the given total size whose header region is well-formed as far as it goes. Sizes below headerBytes are truncations of a valid header; sizes at or above it are a valid header plus that many payload bytes.
the header's documented width, in bytes.