Pin Markdownlint version

[rlippmann]

What changed

• pins scripts/validate_markdown.sh to markdownlint-cli2@0.23.0 for reproducible Markdownlint CLI resolution
• adds a narrow MD004: false override for tests/fixtures/engine-regression/structured/README.md so the synchronized fixture content stays unchanged

Why

• this is a post-merge reproducibility follow-up to the earlier Markdownlint rollout
• synchronized fixture content is unchanged, and there are no runtime behavior or package-version changes

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)

Additional validation run:

• ./scripts/validate_markdown.sh — passed