[plenarius]

Sets PORTED_FROM = "v4.12.0", and the corpus parity line in python/README.md.

Corpus parity on the game's 32,832 models was run on #107's branch. The code the Python port mirrors is the same at this commit: #106 changes only variants code, which is not ported, and #107's last amend changed a Go comment.

• decompile: 25,597 binary models identical; Go and Python give the same warnings and notes on all of them;
• parse: 7,235 ASCII models identical, and 3,045 mutations identical or refused by both;
• compile: 32,832 identical, and the same as cleanmodels compile --cli.

On merged main (957c1b8), go test ./... and the three fixture parity scripts pass.

Release notes draft: ~/dev/nwn/nwmain-analysis/release-notes-v4.12.0.md.