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/adapter-dom.sx

@@ -952,7 +952,7 @@
                 (dom-set-attr el k (str (dict-get attrs k))))
                 extra-keys)
               ;; Flush any newly collected CSS rules to live stylesheet
-              (flush-cssx-to-dom))
+              (run-post-render-hooks))
             ;; No longer a spread — clear tracked state
             (do
               (set! prev-classes (list))