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/types.py

@@ -278,9 +278,33 @@ class ActionDef:
         return f"<action:{self.name}({', '.join(self.params)})>"
 
 
+# ---------------------------------------------------------------------------
+# StyleValue
+# ---------------------------------------------------------------------------
+
+@dataclass(frozen=True)
+class StyleValue:
+    """A resolved CSS style produced by ``(css :flex :gap-4 :hover:bg-sky-200)``.
+
+    Generated by the style resolver. The renderer emits ``class_name`` as a
+    CSS class and registers the CSS rule for on-demand delivery.
+    """
+    class_name: str           # "sx-a3f2c1"
+    declarations: str         # "display:flex;gap:1rem"
+    media_rules: tuple = ()   # ((query, decls), ...)
+    pseudo_rules: tuple = ()  # ((selector, decls), ...)
+    keyframes: tuple = ()     # (("spin", "@keyframes spin{...}"), ...)
+
+    def __repr__(self):
+        return f"<StyleValue {self.class_name}>"
+
+    def __str__(self):
+        return self.class_name
+
+
 # ---------------------------------------------------------------------------
 # Type alias
 # ---------------------------------------------------------------------------
 
 # An s-expression value after evaluation
-SExp = int | float | str | bool | Symbol | Keyword | Lambda | Macro | Component | HandlerDef | RelationDef | PageDef | QueryDef | ActionDef | list | dict | _Nil | None
+SExp = int | float | str | bool | Symbol | Keyword | Lambda | Macro | Component | HandlerDef | RelationDef | PageDef | QueryDef | ActionDef | StyleValue | list | dict | _Nil | None