Implement deref-as-shift: ReactiveResetFrame, DerefFrame, continuation capture

frames.sx: ReactiveResetFrame + DerefFrame constructors,
kont-capture-to-reactive-reset, has-reactive-reset-frame?.
cek.sx: deref as CEK special form, step-sf-deref pushes DerefFrame,
reactive-shift-deref captures continuation as signal subscriber,
ReactiveResetFrame in step-continue calls update-fn on re-render.
adapter-dom.sx: cek-reactive-text/cek-reactive-attr using cek-run
with ReactiveResetFrame for implicit DOM bindings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/adapter-dom.sx

@@ -1107,6 +1107,48 @@
             (reset! sig (dom-get-prop el "value"))))))))
 
 
+;; --------------------------------------------------------------------------
+;; CEK-based reactive rendering (opt-in, deref-as-shift)
+;; --------------------------------------------------------------------------
+;;
+;; When enabled, (deref sig) inside a reactive-reset boundary performs
+;; continuation capture: "the rest of this expression" becomes the subscriber.
+;; No explicit effect() wrapping needed for text/attr bindings.
+
+(define *use-cek-reactive* true)
+(define enable-cek-reactive! (fn () (set! *use-cek-reactive* true)))
+
+;; cek-reactive-text — create a text node bound via continuation capture
+(define cek-reactive-text :effects [render mutation]
+  (fn (expr env)
+    (let ((node (create-text-node ""))
+          (update-fn (fn (val)
+                       (dom-set-text-content node (str val)))))
+      (let ((initial (cek-run
+                       (make-cek-state expr env
+                         (list (make-reactive-reset-frame env update-fn true))))))
+        (dom-set-text-content node (str initial))
+        node))))
+
+;; cek-reactive-attr — bind an attribute via continuation capture
+(define cek-reactive-attr :effects [render mutation]
+  (fn (el attr-name expr env)
+    (let ((update-fn (fn (val)
+            (cond
+              (or (nil? val) (= val false)) (dom-remove-attr el attr-name)
+              (= val true) (dom-set-attr el attr-name "")
+              :else (dom-set-attr el attr-name (str val))))))
+      ;; Mark for morph protection
+      (let ((existing (or (dom-get-attr el "data-sx-reactive-attrs") ""))
+            (updated (if (empty? existing) attr-name (str existing "," attr-name))))
+        (dom-set-attr el "data-sx-reactive-attrs" updated))
+      ;; Initial render via CEK with ReactiveResetFrame
+      (let ((initial (cek-run
+                       (make-cek-state expr env
+                         (list (make-reactive-reset-frame env update-fn true))))))
+        (invoke update-fn initial)))))
+
+
 ;; --------------------------------------------------------------------------
 ;; render-dom-portal — render children into a remote target element
 ;; --------------------------------------------------------------------------