Hash compiled CSS output in incremental build to detect Sass partial changes

[astro-factory]

Changes

• Pages using experimental.incrementalBuild no longer restore with stale CSS when only a Sass partial (or other preprocessor dependency) changes. Previously, hashModules() read the raw source of the entry stylesheet from disk, so edits to @use/@import-ed partials went undetected and cached pages were served with a 404 stylesheet or stale inline styles.
• A transform hook added to pluginIncremental captures the fully compiled CSS output (after Vite's CSS plugin resolves all preprocessor imports) and stores it in a map keyed by module id. hashModules() now uses this compiled output instead of the raw source, naturally covering Sass partials, additionalData, and any other preprocessor-level inputs.
• Pages restored by experimental.incrementalBuild no longer link a raw /_astro/ original image that the new build deleted. A restored page never renders, so its raw .src image references were absent from the global preservation set and the asset pipeline removed the originals as optimization-only. Raw references are now recorded per page in the incremental manifest and replayed when the page is restored.

Testing

• New unit test cases in packages/astro/test/units/build/plugin-incremental.test.ts cover the scenario where a Sass partial changes while the entry stylesheet source stays the same, verifying that the dependency hash changes and the page is re-rendered rather than restored.

Docs

• No docs update needed; this is a bug fix for existing experimental.incrementalBuild behavior with no API changes.

Closes #17974
Closes #17993

[changeset-bot]

🦋 Changeset detected

Latest commit: 299f18c

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

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[codspeed]

Merging this PR will not alter performance

✅ 21 untouched benchmarks

---

_{Comparing factory/fix-17974 (299f18c) with main (0e5478d)}