[Tenkei]

CI compatibility fixes: This PR also corrects two existing catalog mappings that fail on the base branch: Zettelkasten's current theme identifier is zettelkasten, and mkdocs_graphviz is a MkDocs plugin rather than a Markdown extension. The latter entry's label is updated to plugin accordingly. The same errors occur in the main-branch CI run before this PR.

Closes #453.

What kind of change does this PR introduce?

• Add a project
• Update a project
• Remove a project
• Add or update a category
• Change configuration
• Documentation
• Other, please describe:

Description:

Add Pixel Lab, a retro-inspired MkDocs theme with pixel borders, bold colors, and arcade-style shadows, to the Theming category.

• Repository: https://github.com/trained-by-humans/mkdocs-pixel-lab
• PyPI: https://pypi.org/project/mkdocs-pixel-lab/ (current release: 0.2.0)
• Documentation and live demo: https://trained-by-humans.github.io/mkdocs-pixel-lab/
• Theme identifier: pixel-lab
• Bundled optional plugins: pixel-lab/metadata, pixel-lab/git-lastmod, and pixel-lab/robots

I maintain Pixel Lab. The entry includes its PyPI package, theme and plugin identifiers, and documentation homepage so the catalog and its consumers can discover them.

Validation:

• Ran the unmodified check_projects.py against the new entry, including installation of the published package and verification of all listed entry points.
• Built a minimal site in strict mode using the published 0.2.0 package, search, and all three bundled plugins.
• Checked for duplicate project/package/theme/plugin identifiers. The changes comprise one new project and the two existing entry corrections described above.
• Ran the unmodified check_projects.py against Zettelkasten, MkDocs Graphviz, and Pixel Lab to verify their current published entry points after the corrections.
• Verified the live documentation/demo responds successfully.
• git diff --check passes.

Checklist:

• I have read the contribution guidelines.
• I have not modified the README.md file. The project is added only to projects.yaml.