fix: limit long-title exemption to title nodes

[w3lld1]

Summary

• restrict allow-long-titles to actual title and subtitle nodes instead of entire section nodes
• preserve D001 checks for continuation lines inside section paragraphs
• add regression coverage based on the reported clang-tidy document shape

Validation

• tox -e py,lint (32 tests passed; coverage threshold, Ruff, mypy, pylint, and all pre-commit hooks passed)
• git diff --check

Fixes #195

[w3lld1]

CI note: the two failing paths appear to be infrastructure failures before the project checks run:

• pre-commit.ci: pyupgrade crashes under Python 3.14 with TypeError: cannot use a bytes pattern on a string-like object
• GitHub Actions prepare: coactions/dynamic-matrix@v4 fails while building its container because cffi cannot find cc

The repository's local tox -e py,lint run passes (32 tests, coverage threshold, Ruff, mypy, pylint, and all pre-commit hooks). Read the Docs and the ack check are green.