fix(css): run final minification on merged CSS output

[sxzz]

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

Description

Previously, LightningCSS minification was only applied per-file during the transform phase. When multiple CSS files share common imports (e.g. common.scss), the concatenated output contained duplicate rules that were never deduplicated. This adds a final LightningCSS transform() pass on the merged CSS in CssPostPlugin (similar to Vite's finalizeCss()), allowing cross-file rule deduplication when css.minify is enabled.

Linked Issues

fixes #851

Additional context

Only packages/css/src/post.ts is changed. The final minification pass runs in both splitting and non-splitting code paths.

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