Remove unrequested additions

[LunaStev]

Summary

Remove the unrequested documentation files, dedicated fixture checker, and README/contributor requirements introduced by #486, at the maintainer's request.

Validation

• Lexer and parser tests pass after removal.
• cargo fmt --all --check and git diff --check pass.
• The resulting tree matches the tested cleanup tree exactly.
• Compiler fixes and their ordinary regression tests remain intact.

Checklist

• Commit includes DCO sign-off.
• Removed files have no remaining references in tracked source.
• Changes are limited to the requested cleanup.