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/sx/templates/layout.sx

@@ -82,6 +82,7 @@
     (div :class "block md:hidden text-md font-bold"
       (when auth-menu auth-menu))))
 
+; @css bg-sky-400 bg-sky-300 bg-sky-200 bg-sky-100
 (defcomp ~menu-row-sx (&key id level colour link-href link-label link-label-content icon
                              hx-select nav child-id child oob external)
   (let* ((c (or colour "sky"))