[eslint-plugin] Add gap auto-fix for legacy-expand-shorthands and skip single-value gap in valid-shorthands

[abhakat]

What changed / motivation ?

Two changes to the eslint-plugin's gap handling:

1. Add gap to shorthandExpansionMap in valid-styles — enables the auto-fixer to expand gap to rowGap/columnGap when styleResolution: 'legacy-expand-shorthands' or banPropsForLegacy is active. No impact on users who don't opt into legacy mode — the auto-fix path is already gated behind isLegacyExpandShorthands.

2. Skip single-value gap in valid-shorthands — gap: 10px is a valid CSS property, not a multi-value shorthand. Previously, the rule would flag it and expand to rowGap: 10px + columnGap: 10px, which is unnecessary. Multi-value gap (e.g., gap: 10px 20px) still correctly expands. gridGap single-value expansion is unchanged since gridGap is a deprecated property name.

Linked PR/Issues

n/a

Additional Context

All 546 eslint-plugin tests pass. 5 single-value gap test cases moved from invalid to valid in the valid-shorthands test suite.

Pre-flight checklist

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

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
stylex	Skipped		May 14, 2026 11:22pm

[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.oehC56qwYQ /tmp/tmp.VxXfxeESri

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	514	525	1.02	+
· complex create	63	63	1.00
babel-plugin: stylex.createTheme
· basic themes	415	412	0.99	-
· complex themes	32	33	1.03	+
Done in 0.08s.
Done in 0.32s.

[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.8nze1SA3b2 /tmp/tmp.jFia6anozs

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,535	1,535	1.00
· minified	5,166	5,166	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	99,757	99,757	1.00
· minified	748,850	748,850	1.00
Done in 0.08s.
Done in 0.34s.