A declared 16/32-bit encoding over BOM-less 8-bit bytes is rejected; a
BOM followed by a declaration outside its family is rejected, reading
the declaration through the BOM's own byte pattern; and an external
entity whose text declaration names a higher XML version than the
document is refused (errata E38). The eduni not-wf baseline is empty.
Reader and decoder helpers split for the length caps.
Closes#137
