Fix Typst label handling in diff output

[kjgoodrick]

Summary

Fix Typst label handling in generated diff output.

This change treats Typst labels like <sample-anchor> as atomic syntax during
inline diffing. It prevents diff markup from being inserted inside label syntax,
and keeps unchanged labels outside added/deleted wrappers so references can
continue to resolve.

Closes #7.

Changes

• Parse labels at the start of a paragraph as their own block.
• Treat label-only blocks as atomic so they are not word-diffed internally.
• Tokenize <label> syntax as a single token during mixed-granularity diffing.
• Render inserted labels bare, and suppress deleted label spans to avoid invalid
  or duplicate labels.
• Add regression tests for renamed labels and unchanged labels near inserted
  content.