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

@@ -1509,33 +1509,9 @@ CEK_FIXUPS_JS = '''
     return cekValue(state);
   };
 
-  // Expose spec functions so evaluated SX code can use them.
-  // Type inspection (platform interface from boundary.sx)
-  PRIMITIVES["type-of"] = typeOf;
-  PRIMITIVES["symbol-name"] = symbolName;
-  PRIMITIVES["keyword-name"] = keywordName;
-  PRIMITIVES["callable?"] = isCallable;
-  PRIMITIVES["lambda?"] = isLambda;
-  PRIMITIVES["lambda-name"] = lambdaName;
-  PRIMITIVES["component?"] = isComponent;
-  PRIMITIVES["island?"] = isIsland;
-  PRIMITIVES["make-symbol"] = function(n) { return new Symbol(n); };
-
-  // Parser (from parser.sx)
-  PRIMITIVES["sx-serialize"] = sxSerialize;
-
-  // CEK machine (from cek.sx/frames.sx)
-  PRIMITIVES["make-cek-state"] = makeCekState;
-  PRIMITIVES["cek-step"] = cekStep;
-  PRIMITIVES["cek-run"] = cekRun;
-  PRIMITIVES["cek-terminal?"] = cekTerminal_p;
-  PRIMITIVES["cek-value"] = cekValue;
-  PRIMITIVES["eval-expr-cek"] = evalExprCek;
-
-  // Render (from adapter-html.sx / render.sx)
+  // Synthetic primitives — not direct spec defines but needed by evaluated code
   PRIMITIVES["is-html-tag?"] = function(n) { return HTML_TAGS.indexOf(n) >= 0; };
-
-  // Environment (for creating eval contexts)
+  PRIMITIVES["make-symbol"] = function(n) { return new Symbol(n); };
   PRIMITIVES["make-env"] = function() { return merge(componentEnv, PRIMITIVES); };
 '''