D021 - There is a second package magic, and it is named and refused rather than guessed at

\x7FFIH alongside \x7FCNT. The console's installer accepts both; selfish-pkg reads only the second, and now says so with its own error.

Two failure modes were available and both are worse than an error:

• Reporting it as not-a-package. False about a file that works, and the sort of thing that sends somebody looking for a fault in their package rather than in the tool.
• Assuming the same layout. The header is big-endian, so reading a count and a table offset out of the wrong header still yields a count and a table offset. Entries would be produced. None of it would be right and none of it would be detectably wrong - which is precisely the shape principle 5 exists to prevent.

The magic is in data/pkg-format.tsv marked layout NOT established. That is the whole entry: knowing a format exists is worth recording separately from knowing how to read it.