diff: keep scanning after equal path-limited siblings

[gaoflow]

Summary

• continue scanning sibling keys when a path-limited value is equal instead of returning from the whole recursion frame
• add a regression test for wildcard path limits preserving nested dictionary additions

Closes #188.

Validation

• python -m pytest tests/test_dictdiffer.py::DictDifferTests::test_path_limit_addition -q -o addopts=''
• python -m pytest tests/test_dictdiffer.py -q -o addopts=''
• python -m pytest -q -o addopts=''
• python -m compileall dictdiffer tests
• git diff --check
• python -m pycodestyle --ignore=E501,W504 dictdiffer/__init__.py tests/test_dictdiffer.py

Note: the repository default pytest addopts require optional pytest lint plugins that are not installed in this local environment, so the test commands above clear addopts for behavioral validation.

This pull request was prepared with the assistance of AI, under my direction and review.