Fix home-stepper paren balance in rebuild-preview

rebuild-preview had one extra close paren that closed the outer
(when container) prematurely, pushing do-back and build-code-dom
out of the letrec scope. Result: "Undefined symbol: build-code-dom".

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/home-stepper.sx

@@ -221,14 +221,13 @@
            (swap! step-idx inc)
            (update-code-highlight)))))
        (rebuild-preview (fn (target)
-         ;; Rebuild preview DOM directly from steps, without replaying do-step
          (let ((container (get-preview)))
            (when container
              (dom-set-prop container "innerHTML" "")
              (let ((expr (steps-to-preview (deref steps) target)))
                (when expr
-                 (let ((rendered (render-to-dom expr (get-render-env nil) nil)))
-                   (when rendered (dom-append container rendered)))))
+                 (let ((dom (render-to-dom expr (get-render-env nil) nil)))
+                   (when dom (dom-append container dom)))))
              (set-stack (list container))))))
        (do-back (fn ()
          (when (> (deref step-idx) 0)