Add on-demand CSS: registry, pre-computed component classes, header compression

- Parse tw.css into per-class lookup registry at startup
- Pre-scan component CSS classes at registration time (avoid per-request regex)
- Compress SX-Css header: 8-char hash replaces full class list (LRU cache)
- Add ;@css comment annotation for dynamically constructed class names
- Safelist bg-sky-{100..400} in Tailwind config for menu-row-sx dynamic shades
- Client sends/receives hash, falls back gracefully on cache miss

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/browser/templates/_types/root/_head.html

@@ -1,15 +1,8 @@
 <style>
       @media (min-width: 768px) { .js-mobile-sentinel { display:none !important; } }
     </style>
-    <link rel="stylesheet" type="text/css" href="{{asset_url('styles/basics.css')}}">
-    <link rel="stylesheet" type="text/css" href="{{asset_url('styles/cards.css')}}">
-    <link rel="stylesheet" type="text/css" href="{{asset_url('styles/blog-content.css')}}">
-    
     <meta name="csrf-token" content="{{ csrf_token() }}">
-    <script src="https://cdn.tailwindcss.com?plugins=typography"></script>
-    <link rel="stylesheet" href="{{asset_url('fontawesome/css/all.min.css')}}">
-    <link rel="stylesheet" href="{{asset_url('fontawesome/css/v4-shims.min.css')}}">
-    <link href="https://unpkg.com/prismjs/themes/prism.css" rel="stylesheet" />
+    <style id="sx-css">{{ sx_css_all() }}</style>
     <script src="https://unpkg.com/prismjs/prism.js"></script>
     <script src="https://unpkg.com/prismjs/components/prism-javascript.min.js"></script>
     <script src="https://unpkg.com/prismjs/components/prism-python.min.js"></script>