The ten single-byte ANSI pages join as a second enum alongside
DosEncoding, mirroring its API. Errors now carry a public Encoding
wrapper (Dos | Windows) so both families share the single-byte codec
functions; Encoding::from_code_page resolves a raw code page number
across families, DOS first.
data/ is reorganized into pc/, windows/, and bestfit/, and the
generated single-byte tables split into dos.rs and ansi.rs. Microsoft
publishes an identical Thai table for both families, so DosEncoding::
Cp874 and WindowsEncoding::Cp874 agree byte-for-byte (pinned by test).
