fix: recursively expand background-position in multi-layer backgrounds

[srobinson]

BREAKING CHANGE: background-position now fully expands to -x and -y even in multi-layer backgrounds

Before:
background: url(a.png) center, url(b.png) left top
→ background-position: center, left top (shorthand)

After:
background: url(a.png) center, url(b.png) left top
→ background-position-x: center, left
→ background-position-y: center, top (longhands)

This fixes integration with @b/values which only supports longhands.

Changes:

• Remove isMultiLayer check in expand.ts
• Remove unused import hasTopLevelCommas
• Update expandBackgroundPosition to handle comma-separated layers
• Split each layer, expand individually, rejoin with commas
• Update test expectations to match new behavior
• Fix invalid-cases test expectations

[Copilot]

Pull Request Overview

This PR fixes recursive expansion of background-position in multi-layer backgrounds by removing the guard that prevented expansion of comma-separated values and updating the expandBackgroundPosition function to handle multi-layer values.

• Removes the isMultiLayer check in src/core/expand.ts to allow recursive expansion of all shorthand properties, including those with comma-separated values
• Updates expandBackgroundPosition to split comma-separated layers, expand each individually, and rejoin with commas
• Updates test expectations to reflect the new behavior where multi-layer background-position values are now fully expanded to -x and -y longhands

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/core/expand.ts	Removed isMultiLayer guard and unused hasTopLevelCommas import to enable recursive expansion for all properties
src/handlers/background-position/expand.ts	Added multi-layer support using splitLayers, expanding each layer individually and rejoining with commas
test/recursive-expansion.test.ts	Updated test expectations to verify multi-layer positions are now expanded to -x and -y longhands
test/invalid-cases.test.ts	Updated expected output to use backgroundPositionX and backgroundPositionY instead of backgroundPosition
FIX_SUMMARY.md	Added documentation explaining the fix and breaking change

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