[Eugene (OEvgeny)]

Prepares the build for tsdown; no behavior change in the published packages.

Changelog Entry

Changed

• 👷🏻 Build checks and tests accept tsdown output as well as tsup, and the CSS placeholder check also finds template literals, in PR #6039, by @OEvgeny

Description

Some build checks and tests assume today's tsup/esbuild output, so switching to tsdown would fail them, and the css-inject check also misses a placeholder written as a template literal. This PR makes them correct for both tools and moves the Lightning CSS options into a shared file; the emitted CSS is byte-identical.

Design

• The CSS pipeline stays ours (unplugin-lightningcss + injectCSSPlugin) rather than @tsdown/css, so both bundlers can import lightningCSSOptions.mjs. It has one user today; moving it now keeps the tsdown pull request to new configs only.

Specific Changes

• vibe-grep css-inject.yaml: also matches template literals (no source writes one today, but `@--X--@` would survive an unminified build: injectCSSPlugin only replaces "…")
• lightningCSSOptions.mjs: Lightning CSS options and removePreserveReason(), used by esbuildPlugins.mjs
• api/src/boot/middleware.ts: type on the AvatarPolymiddlewareProxyProps re-export
• vibe-grep lightning-css.yaml: accepts :has( > …) as well as :has(> …)
• boot/bundle.html, fluentTheme/simple.html, preact/fluentTheme/simple.html: build tool may be tsup or tsdown

• I have added tests and executed them locally
• I have added .changelog/<PR#>-<slug>.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

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