feat: lazy-load non-critical vendor CSS

[pivaldi]

Motivation

FontAwesome, Fancybox and KaTeX CSS are render-blocking but not needed for
first paint. Loading them eagerly hurts Lighthouse / Core Web Vitals (FCP, LCP)
on pages that don't use them above the fold. This makes those stylesheets
non-render-blocking via opt-in lazy loading.

Changes

• New opt-in config (_config.yml), disabled by default so behavior is
  unchanged unless enabled:

  # Performance optimization
  performance:
    # Lazy-load non-critical CSS (FontAwesome, Fancybox, KaTeX)
    # This improves Lighthouse scores by making these CSS files non-render-blocking
    lazy_css: false

• next_vendors helper (scripts/helpers/engine.js) accepts an options
  object with a lazy flag. When lazy is set and performance.lazy_css
  is enabled, the stylesheet is loaded with the standard preload + onload
  swap technique, plus a <noscript> fallback for JS-disabled clients:

  <link rel="preload" href="…" as="style" onload="this.onload=null;this.rel='stylesheet'">
  <noscript><link rel="stylesheet" href="…"></noscript>

  JS handling is unchanged (it already uses defer). Existing integrity /
  crossorigin attributes are preserved in both the preload and noscript tags.

• Templates opt the three non-critical stylesheets in:

  • layout/_partials/head/head.njk — fontawesome, fancybox_css
  • layout/_third-party/math/katex.njk — katex

Notes

• Off by default → no change for existing users until performance.lazy_css: true.
• Backward compatible: next_vendors(name) still works; the options argument
  is optional.
• Split out from feat: use calculated integrity hashes for local vendor plugins #947 so it can be reviewed independently of the SRI fix.

Testing

• performance.lazy_css: false (default): tags render exactly as before
  (render-blocking <link rel="stylesheet">).
• performance.lazy_css: true: FontAwesome / Fancybox / KaTeX CSS emit the
  preload + onload swap with a <noscript> fallback; styles still apply
  with and without JavaScript.

[CLAassistant]

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[github-actions]

This pull request contains changes to the configuration file. Please make sure the documentation in NexT website is changed or added.
Please edit relevant source files here: https://github.com/next-theme/theme-next-docs/tree/master/source/docs and create a pull request with the changes here: https://github.com/next-theme/theme-next-docs/pulls