Create tests to catch this issue

[jeffpatton1971]

Targeted tests to validate internal links across per-type and single-file outputs. They use synthetic XML so they’re stable and self-contained, and they assert both the link href and that the referenced anchor actually exists in the generated Markdown.

[jeffpatton1971]

What these tests cover

• Per-type mode:
  • Type links target the correct per-type file name.
  • Member links target per-type file + correct member anchor.
  • The referenced member anchor exists.
• Single-file mode:
  • Type links use heading slugs (e.g., #foo).
  • Member links use in-document anchors (IdToAnchor).
  • Both anchors exist in the document.
• Nested generics:
  • The href used for a complex method link exactly matches an emitted anchor, avoiding drift from IdToAnchor logic.