Before, the offset for `FLAGS` in `2mg.ts` was `0x0A`, which is
incorrect according to the spec at:
https://apple2.org.za/gswv/a2zine/Docs/DiskImage_2MG_Info.txt
Now, all of the fields in the 2mg header are described, including
their lengths and any constraints. These constraints are enforced by
`read2MGHeader` and tested by new tests.
