Build: `experimental.lightningCssFeatures` exclude list not honored

This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-22).

Problem

Setting lightningCssFeatures to exclude light-dark() should preserve the CSS function untransformed. vinext still transforms it into var(--lightningcss-light, ...) polyfill output, ignoring the exclude config.

Excluded `light-dark()` still transformed by LightningCSS

Estimated Impact

~1 test failures across the deploy suite.

Affected Test Suites

• test/e2e/app-dir/experimental-lightningcss-features/experimental-lightningcss-features.test.ts

Recommendation

1. Reproduce first in vinext's own test suite. Configure experimental: { lightningCssFeatures: { exclude: ['light-dark'] } } and assert the function survives in the output.

2. Plumb the exclude list into LightningCSS. Read lightningCssFeatures.exclude and pass it as the corresponding exclude bitmask/options when calling LightningCSS.