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/boot.sx

@@ -431,9 +431,8 @@
   (fn ()
     (let ((rules (collected "cssx")))
       (when (not (empty? rules))
-        (let ((style (or (dom-query "#sx-cssx-live")
+        (let ((style (or (dom-query "[data-cssx]")
                          (let ((s (dom-create-element "style" nil)))
-                           (dom-set-attr s "id" "sx-cssx-live")
                            (dom-set-attr s "data-cssx" "")
                            (dom-append-to-head s)
                            s))))