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.

[Copilot]

Pull request overview

This PR improves Typst label handling in typdiff output so generated diffs remain valid Typst and label-based references continue to resolve. It does so by recognizing <label> syntax as atomic during inline diffing, splitting paragraph-start labels into their own blocks, and rendering labels without wrapping them in diff markup.

Changes:

• Parse labels at paragraph start as their own paragraph blocks and treat label-only blocks as atomic (skip word-diffing inside them).
• Tokenize <label> as a single token during mixed-granularity word diffing to prevent diff markup from splitting label syntax.
• Render inserted labels bare and suppress deleted label output; add regression tests covering renamed and unchanged labels near edits.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/lib.rs	Introduces TypstLabel helper and marks label-only paragraph blocks as atomic.
src/diff.rs	Updates mixed tokenizer to treat <label> as an atomic token (via TypstLabel).
src/parse.rs	Splits paragraph-start labels into their own paragraph blocks; adds parser regression test.
src/render.rs	Renders labels bare (and suppresses deleted label output) to avoid invalid/duplicate labels; adds renderer regression test.
tests/integration.rs	Adds integration tests for renamed labels and unchanged label placement relative to added content.

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[kjgoodrick]

Thanks for the review, fixed.

I updated render_spans() so skipped deleted label-only spans no longer clear the previous diff state. I also fixed a related deleted-side rendering case where #ref(<label>, ...) could incorrectly escape the label argument and produce invalid Typst.

Added regression coverage for both fixes, including the following cases:

• skipped deleted labels preserving the diff-call guard
• #ref(<label>, ...) keeping the first label argument intact
• replaced footnote refs rendering correctly on the deleted side