As it turns out the thing I marked as "header size" is more likely just a discarded revision.
Anyways, to detail this: pak version 5 adds fields to the header for sector alignment. The reader now handles this and reads the sectoralignment TOC entry, which now allows STREAMS (and any other packages with this set?) to extract properly.
Woohoo.
................. fuck
This really needs to be cleaned up before I'm willing to call it "good" but ultimately the API changes here needed to be done anyhow
