fix(css): use unique key for cssEntriesMap to prevent same-basename collision

[teee32]

Problem

When two or more CSS-only entry points in different directories share the same basename (e.g. store/skins/style.scss and store3/skins/style.scss), Vite's CSS post-processing incorrectly cross-links them in the build manifest. This causes both stylesheets to be loaded at runtime when only one was requested, leading to unintended style overrides.

Reported in #22013.

Root Cause

cssEntriesMap in packages/vite/src/node/plugins/css.ts uses chunk.name as the Map key. When entries share a basename, they share a chunk.name, so the second set() overwrites the first. Later in generateBundle, the lookup returns the wrong referenceId, causing one entry's CSS file to be transferred as a dependency of the other.

cssEntriesMap.set(chunk.name, referenceId)   // collision when basenames match
//                ^^^^^^^^^^

Change

Replace chunk.name with originalFileName (the normalized root-relative path produced by getChunkOriginalFileName, which is already computed in the same scope) as the Map key. This is always unique for distinct entry chunks. Falls back to chunk.name when facadeModuleId is unavailable.

The same key strategy is applied on both the write path (renderChunk) and the read path (generateBundle).

Why This Fix

• originalFileName is derived from chunk.facadeModuleId via getChunkOriginalFileName, the same function used by the manifest plugin to identify chunks — so the key is consistent with existing naming conventions.
• The change is minimal (2 locations in css.ts) and does not alter the module type or any public API.
• cssEntriesMap is also consumed by manifest.ts, where the key is passed to Rolldown's nativeManifestPlugin as the CSS entry name. Changing from basename to relative path makes CSS entry naming consistent with JS chunk naming in the manifest.

Risk

• Low. The fallback ?? chunk.name preserves existing behavior when facadeModuleId is absent.
• No change to public API or plugin hook signatures.
• The manifest name field for CSS entries changes from basename to relative path, which is consistent with how JS chunks are already named.

Validation

• Added a regression test: two CSS entry files (a/index.css, b/index.css) sharing the basename index.css in different directories, each with a distinct asset. The test verifies the manifest does not cross-link their assets.
• All 54 existing build tests pass.
• Lint and prettier pass via pre-commit hooks.

Closes #22013

Co-Authored-By: Codex noreply@openai.com
Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

[sapphi-red]

I've updated the test to ensure isEntry is set and the test failed. To fix that, I made a change on the native plugin side: rolldown/rolldown#9059
I also changed the code to use chunk.filename instead of originalFileName as that will be simpler.

[sapphi-red]

(I'll merge this once the next rolldown release is out)