Implement CSSX Phase 2: native SX style primitives

Replace Tailwind class strings with native SX expressions:
(css :flex :gap-4 :hover:bg-sky-200) instead of :class "flex gap-4 ..."

- Add style_dict.py: 516 atoms, variants, breakpoints, keyframes, patterns
- Add style_resolver.py: memoized resolver with variant splitting
- Add StyleValue type to types.py (frozen dataclass with class_name, declarations, etc.)
- Add css and merge-styles primitives to primitives.py
- Add defstyle and defkeyframes special forms to evaluator.py and async_eval.py
- Integrate StyleValue into html.py and async_eval.py render paths
- Add register_generated_rule() to css_registry.py, fix media query selector
- Add style dict JSON delivery with localStorage caching to helpers.py
- Add client-side css primitive, resolver, and style injection to sx.js

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

shared/sx/css_registry.py

@@ -147,6 +147,48 @@ def scan_classes_from_sx(source: str) -> set[str]:
     return classes
 
 
+def register_generated_rule(style_val: Any) -> None:
+    """Register a generated StyleValue's CSS rules in the registry.
+
+    This allows generated class names (``sx-a3f2c1``) to flow through
+    the existing ``lookup_rules()`` → ``SX-Css`` delta pipeline.
+    """
+    from .style_dict import CHILD_SELECTOR_ATOMS
+    cn = style_val.class_name
+    if cn in _REGISTRY:
+        return  # already registered
+
+    parts: list[str] = []
+
+    # Base declarations
+    if style_val.declarations:
+        parts.append(f".{cn}{{{style_val.declarations}}}")
+
+    # Pseudo-class rules
+    for sel, decls in style_val.pseudo_rules:
+        if sel.startswith("::"):
+            parts.append(f".{cn}{sel}{{{decls}}}")
+        elif "&" in sel:
+            # group-hover pattern: ":is(.group:hover) &" → .group:hover .sx-abc
+            expanded = sel.replace("&", f".{cn}")
+            parts.append(f"{expanded}{{{decls}}}")
+        else:
+            parts.append(f".{cn}{sel}{{{decls}}}")
+
+    # Media-query rules
+    for query, decls in style_val.media_rules:
+        parts.append(f"@media {query}{{.{cn}{{{decls}}}}}")
+
+    # Keyframes
+    for _name, kf_rule in style_val.keyframes:
+        parts.append(kf_rule)
+
+    rule_text = "".join(parts)
+    order = len(_RULE_ORDER) + 10000  # after all tw.css rules
+    _REGISTRY[cn] = rule_text
+    _RULE_ORDER[cn] = order
+
+
 def registry_loaded() -> bool:
     """True if the registry has been populated."""
     return bool(_REGISTRY)