Fix duplicate sx-cssx-live style tags

Cache the style element reference in _cssx-style-el so flush-cssx-to-dom
never creates more than one. Previous code called dom-query on every
flush, which could miss the element during rapid successive calls,
creating duplicates.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/sx/ref/platform_js.py

@@ -1537,6 +1537,10 @@ CEK_FIXUPS_JS = '''
 
   // Environment (for creating eval contexts)
   PRIMITIVES["make-env"] = function() { return merge(componentEnv, PRIMITIVES); };
+
+  // Render hooks (from boot.sx)
+  PRIMITIVES["register-pre-render-hook"] = registerPreRenderHook;
+  PRIMITIVES["register-post-render-hook"] = registerPostRenderHook;
 '''