fix: keep zero-runtime stylesheet modules out of client treeshaking

[DylanPiercey]

The client build defaults every module to side-effect free, keeping only .marko files, styles, assets, and bare-import subgraphs. A zero-runtime stylesheet module (*.css.ts / *.css.js, as used by vanilla-extract) looks pure — it only exports class name strings — so when nothing in the client bundle references those exports, the module is dropped and its stylesheet is silently never emitted. The page renders unstyled with no build warning.

This treats those modules as side-effectful by path, alongside styles and assets.

[changeset-bot]

🦋 Changeset detected

Latest commit: ced6867

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

This PR includes changesets to release 1 package

Name	Type
@marko/vite	Patch

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: Organization UI

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Run ID: 82923498-014f-4a56-9c0f-293e8dd23461

📥 Commits

Reviewing files that changed from the base of the PR and between a435f52 and ced6867.

📒 Files selected for processing (1)

• src/index.ts

🚧 Files skipped from review as they are similar to previous changes (1)

• src/index.ts

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Walkthrough

The change classifies .css.js and .css.ts stylesheet modules as side-effectful in isSideEffectFile. The browser-side-effects fixture adds a server-only stylesheet-module import and records client-side loading. A patch changeset documents the behavior.

Merge Risk: ⚪ Minimal · up to ced68

This localized change keeps stylesheet modules from being removed during client tree-shaking; no actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: preventing zero-runtime stylesheet modules from client tree-shaking.
Description check	✅ Passed	The description directly explains the tree-shaking issue and the fix for zero-runtime stylesheet modules.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch dpiercey-ws-vanilla-extract-test-2rvyxh

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