[eslint-plugin] Add border shorthand expansion to stylex-valid-shorthands

[abhakat]

Expand border: '<width> <style> <color>' into borderWidth, borderStyle, and borderColor
longhands. Reuses the existing expandBorderSideShorthand and parseBorderParts helpers.
Also adds borderStart → borderInlineStart and borderEnd → borderInlineEnd as legacy
name renames.

What changed / motivation ?

The rule already handled borderTop, borderRight, borderBottom, borderLeft but was missing
the base border shorthand. Incomplete borders (e.g. border: '1px solid' with missing color)
correctly report an error without autofix (CANNOT_FIX).

• border: '1px solid red' → borderWidth: '1px' + borderStyle: 'solid' + borderColor: 'red'
• border: 'none' → passes through (single value, no expansion needed)
• border: '1px solid' → error, no autofix (missing color)
• borderStart → borderInlineStart (legacy rename)
• borderEnd → borderInlineEnd (legacy rename)

Linked PR/Issues

Follow-up to #1477

Additional Context

Added tests covering: full 3-part expansion, rgba() colors, !important propagation,
incomplete borders (CANNOT_FIX), single-value passthrough, separate legacy rename tests.

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
stylex	Skipped		Apr 3, 2026 7:20am

[github-actions]

workflow: benchmarks/perf

Comparison of performance test results, measured in operations per second. Larger is better.
yarn workspace v1.22.22
yarn run v1.22.22
$ node ./compare.js /tmp/tmp.MtmDeTNzZj /tmp/tmp.iTRFmmR9K6

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	541	540	1.00	-
· complex create	66	67	1.02	+
babel-plugin: stylex.createTheme
· basic themes	430	441	1.03	+
· complex themes	34	34	1.00
Done in 0.08s.
Done in 0.34s.

[github-actions]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.
yarn workspace v1.22.22
yarn run v1.22.22
$ node ./compare.js /tmp/tmp.JwVKfHaihF /tmp/tmp.0nYL23TfmM

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,441	1,441	1.00
· minified	4,386	4,386	1.00
@stylexjs/stylex/lib/cjs/inject.js
· compressed	1,793	1,793	1.00
· minified	4,915	4,915	1.00
benchmarks/size/.build/bundle.js
· compressed	496,650	496,650	1.00
· minified	4,847,840	4,847,840	1.00
benchmarks/size/.build/stylex.css
· compressed	99,653	99,653	1.00
· minified	747,850	747,850	1.00
Done in 0.09s.
Done in 0.34s.