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/tests/test_components.py

@@ -237,7 +237,7 @@ class TestBaseShell:
         assert "<html" in html
         assert "<title>Test</title>" in html
         assert "<p>Hello</p>" in html
-        assert "tailwindcss" in html
+        assert "tw.css" in html
 
 
 class TestErrorPage: