Fix CSS url() asset references

[Luca Schneider (Mad-Kat)]

Fixes #513

Root cause: The \0 prefix on virtual module IDs prevents Vite's CSS pipeline from determining the correct directory for url() resolution.

Fix: Drop the \0 prefix and use a .yak.css suffix instead. resolveId now returns /abs/path/Component.tsx.yak.css, giving Vite the correct directory context to resolve relative url() paths. This follows the same convention as vanilla-extract, Linaria, and Pigment CSS. Vite docs confirms that \0 isn't needed for file-derived modules.

Cross-file url() references: A second issue exists with url() paths inside cross-file mixins. When a mixin is inlined into a consuming component, relative url() paths still point relative to the mixin's original directory, not the consuming file's directory. To fix this, relative url() paths are now rewritten during cross-file resolution so they resolve correctly from the consumer's location.

[changeset-bot]

🦋 Changeset detected

Latest commit: aa16a28

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 8 packages

Name	Type
next-yak	Patch
storybook-addon-yak	Patch
next-yak-example	Patch
@example/storybook	Patch
vite-yak-example	Patch
benchmarks	Patch
docs	Patch
yak-swc	Patch

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The base branch was changed.

[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks
⏩ 12 skipped benchmarks¹

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_{Comparing fix/vite-css-url-resolution (fd0a523) with main (2da1696)}

Footnotes

1. 12 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Jan Nicklas (jantimon)]

nextjs with webpack fails because of a bug in mini-css-extract plugin which was resolved by Luca Schneider (@Mad-Kat) in webpack/mini-css-extract-plugin#1161 and merged - so we have to wait for a new next.js release with mini-css-extract-plugin 2.10.2