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/orchestration.sx

@@ -460,7 +460,7 @@
     (sx-process-scripts root)
     (sx-hydrate root)
     (sx-hydrate-islands root)
-    (flush-cssx-to-dom)
+    (run-post-render-hooks)
     (process-elements root)))
 
 
@@ -871,7 +871,7 @@
       (hoist-head-elements-full target)
       (process-elements target)
       (sx-hydrate-elements target)
-      (flush-cssx-to-dom)
+      (run-post-render-hooks)
       (dom-dispatch target "sx:clientRoute"
         (dict "pathname" pathname))
       (log-info (str "sx:route client " pathname)))))