Fix nested class annotations in import-hooked methods

[HanpuLi]

Changes

Fixes #402.

The import hook injects annotation checks into method bodies, but method bodies do not inherit the containing class namespace. An annotation such as value: NestedEnum is valid while the class body is being built, then becomes an unresolved bare name when the instrumented method is called.

Track direct nested class names while transforming the class body and resolve those names through the implicit __class__ closure in method annotations. The class body is pre-scanned, so explicit/forward annotations also work when the referenced nested class is declared later in the class body. Module/global names remain unchanged.

The regression coverage includes the nested Enum case from #402, an invalid value that must still raise TypeCheckError, and a forward reference to a nested enum declared later in the class body. The existing separate @typechecked limitation remains xfailed and documented.

Local verification:

• coverage run -m pytest: 543 passed, 5 skipped, 6 xfailed
• pre-commit run --all-files: passed (including the project's mypy hook)
• sphinx-build -W -n docs <tmpdir>: passed
• Ruff check and format check on changed files
• git diff --check

Checklist

• Changes have tests
• Documentation/changelog updated

[coveralls]

coverage: 94.867% (+0.01%) from 94.853% — HanpuLi:fix/enum-nested-default-annotation into agronholm:master