fix: flatten decorations to prevent stack overflow

[Lakshith7748]

Description

This PR fixes a RangeError: Maximum call stack size exceeded when using many overlapping decorations.

The issue was caused by transformer-decorations creating deeply nested
span
elements for overlapping decorations. When the nesting depth exceeded the stack limit (e.g., > 2500 levels), hast-util-to-html would crash due to recursion.

This fix modifies transformer-decorations to flatten the decoration structure. Instead of wrapping the existing content in a new
span
for every decoration (which creates nesting), the transformer now applies the decoration to each child element individually when possible.

Linked Issues

Fixes #592

Additional context

• Modified packages/core/src/transformer-decorations.ts to apply decorations to individual children instead of creating a wrapper.

• Preserved the behavior of creating a wrapper for empty decorations (insertion points).

• Updated snapshots in packages/core/test/decorations.test.ts to reflect the flattened structure.

• Verified with a reproduction test case involving 5000 overlapping decorations.

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[codecov]

Codecov Report

❌ Patch coverage is 43.33333% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.48%. Comparing base (5ddea95) to head (be2fbbd).

Files with missing lines	Patch %	Lines
packages/core/src/transformer-decorations.ts	43.33%	51 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1182      +/-   ##
==========================================
- Coverage   95.08%   94.48%   -0.60%     
==========================================
  Files          91       91              
  Lines        7866     7941      +75     
  Branches     1662     1672      +10     
==========================================
+ Hits         7479     7503      +24     
- Misses        381      432      +51     
  Partials        6        6              

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[antfu]

I think we should provide an option for wrap/flatten style.

And I am not sure if we need to introduce the options to disable the interaction check. Maybe for flatten mode we could just skip it

[antfu]

The current approach is a breaking change, and we would not take it. Refer to the comment above.

[Lakshith7748]

The current approach is a breaking change, and we would not take it. Refer to the comment above.

Thanks for the feedback! I've updated the PR to add a decorationsStyle option instead of making breaking changes.

• 'wrap' (default): Current behavior with intersection validation
• 'flatten': Skips intersection check, allows deeply nested decorations