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/html.py

@@ -24,11 +24,18 @@ rendered as HTML::
 
 from __future__ import annotations
 
+import contextvars
 from typing import Any
 
 from .types import Component, Keyword, Lambda, NIL, Symbol
 from .evaluator import _eval, _call_component
 
+# ContextVar for collecting CSS class names during render.
+# Set to a set[str] to collect; None to skip.
+css_class_collector: contextvars.ContextVar[set[str] | None] = contextvars.ContextVar(
+    "css_class_collector", default=None
+)
+
 
 class _RawHTML:
     """Marker for pre-rendered HTML that should not be escaped."""
@@ -455,6 +462,13 @@ def _render_element(tag: str, args: list, env: dict[str, Any]) -> str:
             children.append(arg)
             i += 1
 
+    # Collect CSS classes if collector is active
+    class_val = attrs.get("class")
+    if class_val is not None and class_val is not NIL and class_val is not False:
+        collector = css_class_collector.get(None)
+        if collector is not None:
+            collector.update(str(class_val).split())
+
     # Build opening tag
     parts = [f"<{tag}"]
     for attr_name, attr_val in attrs.items():