fix: generate correct filename for sass files when dts is enabled

[ocavue]

• This PR contains AI-generated code, but I have carefully reviewed it myself. Otherwise, my PR may be closed.

Description

Linked Issues

Closes #801

Additional context

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commit: d4d8df4

[Copilot]

Pull request overview

Fixes CSS/Sass output naming in splitting mode when declaration generation (dts) is enabled (reported in #801), primarily by adding regression coverage and updating related dependencies/config.

Changes:

• Added new regression tests + snapshots covering css.splitting=true with/without dts, including Sass (.scss) cases.
• Updated CSS option docs to clarify how fileName is used when splitting=false.
• Updated workspace dependencies/config to support Sass in tests and to pick up a patched rolldown-plugin-dts.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/css.test.ts	Adds regression tests for splitting + dts + Sass scenarios and adjusts the basic fixture naming.
tests/snapshots/css/with-splitting-true.snap.md	Snapshot for splitting output (index.css, index.mjs).
tests/snapshots/css/with-dts-true-and-splitting-true.snap.md	Snapshot for splitting output when dts=true (index.css, index.d.mts, index.mjs).
tests/snapshots/css/with-sass-and-splitting-true.snap.md	Snapshot for Sass + splitting output (index.css, index.mjs).
tests/snapshots/css/with-sass-and-dts-true-and-splitting-true.snap.md	Snapshot for Sass + dts=true + splitting output (index.css, index.d.mts, index.mjs).
src/features/css/index.ts	Docstring clarification for splitting and fileName.
pnpm-workspace.yaml	Adds sass to catalogs and switches rolldown-plugin-dts to a URL-based specifier; updates pnpm trust policy config.
package.json	Adds sass to devDependencies via workspace catalog.
packages/css/package.json	Adds optional peer deps for sass and sass-embedded.
pnpm-lock.yaml	Lockfile updates for Sass deps and the URL-based rolldown-plugin-dts specifier.

Files not reviewed (1)

• pnpm-lock.yaml: Language not supported

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[Copilot]

The rolldown-plugin-dts catalog entry is switched from a semver range to a pkg.pr.new URL. This is an operational/release risk (non-registry, potentially ephemeral artifact) and makes dependency provenance/versioning harder to track. Consider reverting to a published version (or pinning via a proper release/tag), and if this is only needed for CI verification, keep it as a temporary local override rather than in the prod catalog.

Suggested change

	rolldown-plugin-dts: https://pkg.pr.new/rolldown-plugin-dts@200
	rolldown-plugin-dts: ^1.0.0-rc.7

[Copilot]

sass and sass-embedded are added as optional peer dependencies with a "*" range. Using an unbounded peer range can allow incompatible major versions and makes the supported version policy unclear. Consider using a constrained semver range (consistent with the other peers here) based on the versions actually supported/tested.

Suggested change

	"sass": "*",
	"sass-embedded": "*",
	"sass": "^1.0.0",
	"sass-embedded": "^1.0.0",