feat(css): default css.transformer to lightningcss

[sxzz]

Summary

• Change the default css.transformer from 'postcss' to 'lightningcss'
• Lightning CSS handles @import via bundleAsync() with better performance and no PostCSS dependency required
• Edge-case tests (cycles, duplicates, @charset, external imports, anonymous layer, layer wrapping) now explicitly use transformer: 'postcss' since they rely on postcss-import-specific behavior
• URL fragment tests remain .fails (Lightning CSS also can't handle them)
• Updated all documentation (EN, ZH, skills reference) to reflect new default

Test plan

• All 83 CSS tests pass
• Issue Incorrect CSS output #668 snapshot updated
• TypeScript type check passes
• ESLint passes

🤖 Generated with Claude Code

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