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>

sx/sxc/init-client.sx.txt

@@ -1,5 +1,5 @@
 ;; ---------------------------------------------------------------------------
-;; SX app boot — styles and behaviors injected on page load
+;; SX app boot — styles, behaviors, and post-render hooks
 ;;
 ;; Replaces inline_css and init_sx from Python app config.
 ;; Called as a data-init script on every page.
@@ -11,6 +11,25 @@
 (collect! "cssx" "@keyframes sxJiggle{0%,100%{transform:translateX(0)}25%{transform:translateX(-.5px)}75%{transform:translateX(.5px)}}")
 (collect! "cssx" "a.sx-request{animation:sxJiggle .3s ease-in-out infinite}")
 
+;; CSSX flush hook — inject collected CSS rules into a <style> tag.
+;; The spec calls (run-post-render-hooks) after hydration/swap/mount.
+;; This is the application's CSS injection strategy.
+(console-log "init-client: registering cssx flush hook, type:" (type-of (fn () nil)))
+(register-post-render-hook
+  (fn ()
+    (console-log "cssx flush: running, rules:" (len (collected "cssx")))
+    (let ((rules (collected "cssx")))
+      (when (not (empty? rules))
+        (let ((style (or (dom-query "[data-cssx]")
+                         (let ((s (dom-create-element "style" nil)))
+                           (dom-set-attr s "data-cssx" "")
+                           (dom-append-to-head s)
+                           s))))
+          (dom-set-prop style "textContent"
+            (str (or (dom-get-prop style "textContent") "")
+                 (join "" rules))))
+        (clear-collected! "cssx")))))
+
 ;; Nav link aria-selected update on client-side routing
 (dom-listen (dom-body) "sx:clientRoute"
   (fn (e)