Render protected Traversable methods in the reference

[pick7]

Summary

• Explicitly include Traversable's protected extension-interface methods in Sphinx autodoc output.
• Render _get_intermediate_items, _list_traverse, and _traverse without exposing unrelated private members.

Addresses #1848.

Validation

• LC_ALL=C LANG=C make SPHINXBUILD=.venv/bin/sphinx-build -C doc clean html
• Verified all three members are present in doc/build/html/reference.html.
• git diff --check

The local docs build completed successfully with existing Python 3.14 autodoc warnings.

AI agent disclosure

This pull request was prepared and submitted by OpenAI Codex acting as an AI agent through the contributor account.

[Byron]

Thanks, let's try it!