fix: preserve CSS propagation for partial pages imported as components

[0xbejaxer]

Summary

• refine build CSS boundary detection so top-level pages only stop traversal when they are referenced exclusively by Astro's virtual page module
• allow partial pages (export const partial = true) that are imported by other pages to keep propagating transitive component CSS in production builds
• add a regression fixture and build test that verifies transitive CSS from an imported partial is present in index.html output

Validation

• pnpm -C packages/astro exec astro-scripts test "test/partials-css-boundary.test.ts"
• Full PR CI matrix is green (build, lint, test, E2E, platform matrix)

Fixes #16296.

[changeset-bot]

🦋 Changeset detected

Latest commit: f22ad01

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

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[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks

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_{Comparing 0xbejaxer:fix-16296-partial-css-boundary (f22ad01) with main (471a4d6)¹}

Footnotes

1. No successful run was found on main (1f31d6f) during the generation of this report, so 471a4d6 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[0xbejaxer]

Quick update after addressing CI feedback:

• Fixed fixture import path in the new regression setup
• Made the assertion minification-safe (checks for scoped selector presence instead of a specific color literal)
• Kept the core fix unchanged: treat top-level pages as CSS boundaries only when they are imported exclusively by the virtual page module, so imported partial pages continue propagating transitive CSS

All checks are now passing on this branch. Happy to adjust test coverage/assertions further if maintainers prefer a different shape.

[AceCodePt]

Thanks @0xbejaxer !
This is a big help!