PPM filename check-digit calculation may not be correct

https://github.com/jaames/flipnote.js/blob/083ca4ef9e42f09dde03550da4dd1670110a1359/src/parsers/filename/ppm.ts#L36

Seems to work on some samples, but not on others. Wonder if it uses the current console MAC suffix, and not the one from the original filename?

Start with src/parsers/filename/ppm.ts at line 36 and compare filename samples where the check digit works with those where it does not. Investigate whether the calculation uses the current console MAC suffix or the suffix from the original filename. Done means the calculation is correct across the reported samples.

Written by the indexing model from the issue text.