fix: resolve relative markdown links in render hook

[imfing]

Summary

• Resolve local Markdown links like ./page.md, ../page.md, and /docs/page.md through Hugo page lookup before falling back to existing absolute-path, resource, and static handling.
• Resolve relative Markdown paths from the current content file directory so both regular content files and leaf page bundles rewrite to clean Hugo URLs.
• Preserve query strings, fragments, missing links, and external link behavior.
• Add a regression test that builds a temporary Hugo site and verifies regular-page links, leaf-bundle links, absolute Markdown links, missing links, and external links.

Testing

• git diff --check
• npx playwright test tests/render-link.spec.ts
• npm run build
• npm test

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