feat(css): extract CSS pipeline into @tsdown/css package

[sxzz]

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

Description

Split the CSS pipeline into two tiers for better modularity and optional advanced features:

• Main package (tsdown): Provides basic CSS support via a built-in plugin that reads CSS files and emits them as assets. Shared utilities (createCssPostHooks, removePureCssChunks) are exported from tsdown/css.
• @tsdown/css package: Advanced CSS features including Lightning CSS integration, preprocessor support (Sass/Less/Stylus), CSS syntax lowering via css.target, and css.minify.

Key changes:

• Extract shared CSS post-processing hooks and pure CSS chunk handling into tsdown/css export path
• Create @tsdown/css package with Lightning CSS bundleAsync/transform, preprocessor compilation, and target-based syntax lowering
• Auto-detect @tsdown/css and use advanced plugin when installed, fall back to built-in basic plugin otherwise
• Add css.minify and css.target options to CssOptions
• Remove CSS source map code (aligning with Vite, which does not support CSS source maps in build mode)
• Add comprehensive CSS tests (13 tests covering basic, unbundle, dts, splitting, minify, target, @import inlining, deep import chains, empty files)

Linked Issues

relate #472

Additional context

CSS source map support was intentionally omitted — Vite does not emit CSS source maps during build (see vitejs/vite#2830), so we align with that behavior.

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tsdown

pnpm add https://pkg.pr.new/rolldown/tsdown@790 -D

npm i https://pkg.pr.new/rolldown/tsdown@790 -D

yarn add https://pkg.pr.new/rolldown/tsdown@790.tgz -D

create-tsdown

pnpm add https://pkg.pr.new/rolldown/tsdown/create-tsdown@790 -D

npm i https://pkg.pr.new/rolldown/tsdown/create-tsdown@790 -D

yarn add https://pkg.pr.new/rolldown/tsdown/create-tsdown@790.tgz -D

@tsdown/css

pnpm add https://pkg.pr.new/rolldown/tsdown/@tsdown/css@790 -D

npm i https://pkg.pr.new/rolldown/tsdown/@tsdown/css@790 -D

yarn add https://pkg.pr.new/rolldown/tsdown/@tsdown/css@790.tgz -D

@tsdown/exe

pnpm add https://pkg.pr.new/rolldown/tsdown/@tsdown/exe@790 -D

npm i https://pkg.pr.new/rolldown/tsdown/@tsdown/exe@790 -D

yarn add https://pkg.pr.new/rolldown/tsdown/@tsdown/exe@790.tgz -D

tsdown-migrate

pnpm add https://pkg.pr.new/rolldown/tsdown/tsdown-migrate@790 -D

npm i https://pkg.pr.new/rolldown/tsdown/tsdown-migrate@790 -D

yarn add https://pkg.pr.new/rolldown/tsdown/tsdown-migrate@790.tgz -D

commit: 3c7e0bf

[sxzz]

tsdown CSS vs Vite Lib Mode CSS Feature Comparison

(Reposting in English — please ignore the previous Chinese comment)

Implemented

Feature	tsdown	Vite Lib Mode	Notes
CSS extraction/bundling	css.fileName	build.lib.cssFileName	tsdown defaults to style.css
CSS Code Splitting	css.splitting	build.cssCodeSplit	Both default to off in lib mode
CSS Minification	css.minify	build.cssMinify	tsdown: Lightning CSS only; Vite: esbuild/lightningcss
CSS Syntax Lowering	css.target	build.cssTarget	Both use esbuild target format
Preprocessors (Sass/Less/Stylus)	css.preprocessorOptions	css.preprocessorOptions	Aligned, including additionalData
@import inlining	Lightning CSS bundleAsync	postcss-import / Lightning CSS	Different implementation, same result
Lightning CSS passthrough	css.lightningcss	css.lightningcss	Aligned
Pure CSS chunk removal	✅	✅	Logic aligned with Vite

Missing Features

Feature	Vite Config	Priority	Notes
CSS Modules	css.modules	High	*.module.css support including localsConvention, generateScopedName, composes, etc. Commonly used in library development
PostCSS support	css.postcss	High	Tailwind CSS, Autoprefixer, and the broader ecosystem rely on PostCSS. Vite supports both inline config and auto-discovery of config files
CSS url() asset resolution	Built-in	Medium	Path rewriting for url() and image-set(). Lightning CSS handles some of this, but lacks a full asset pipeline
css.transformer selection	css.transformer	Low	Vite supports 'postcss' | 'lightningcss' toggle. tsdown currently uses Lightning CSS only
Preprocessor worker pool	css.preprocessorMaxWorkers	Low	Multi-threaded preprocessor compilation for large projects
CSS inline/URL queries	?inline / ?url	Low	import styles from './foo.css?inline' query parameter support
CSS Source Maps	build.sourcemap	—	Vite doesn't fully support CSS source maps in build mode either (vitejs/vite#2830). Intentionally skipped

Summary

Core CSS features (extraction, splitting, minification, syntax lowering, preprocessors, @import inlining) are aligned with Vite lib mode. The main missing high-priority features are CSS Modules and PostCSS support.