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/frames.sx

@@ -166,6 +166,18 @@
     {:type "dynamic-wind" :phase phase
      :body-thunk body-thunk :after-thunk after-thunk :env env}))
 
+;; ReactiveResetFrame: delimiter for reactive deref-as-shift
+;; Carries an update-fn that gets called with new values on re-render.
+(define make-reactive-reset-frame
+  (fn (env update-fn first-render?)
+    {:type "reactive-reset" :env env :update-fn update-fn
+     :first-render first-render?}))
+
+;; DerefFrame: awaiting evaluation of deref's argument
+(define make-deref-frame
+  (fn (env)
+    {:type "deref" :env env}))
+
 
 ;; --------------------------------------------------------------------------
 ;; 3. Frame accessors
@@ -202,12 +214,35 @@
     ;; Returns (captured-frames remaining-kont).
     ;; captured-frames: frames from top up to (not including) ResetFrame.
     ;; remaining-kont: frames after ResetFrame.
+    ;; Stops at either "reset" or "reactive-reset" frames.
     (define scan
       (fn (k captured)
         (if (empty? k)
           (error "shift without enclosing reset")
           (let ((frame (first k)))
-            (if (= (frame-type frame) "reset")
+            (if (or (= (frame-type frame) "reset")
+                    (= (frame-type frame) "reactive-reset"))
               (list captured (rest k))
               (scan (rest k) (append captured (list frame))))))))
     (scan kont (list))))
+
+;; Check if a ReactiveResetFrame exists anywhere in the continuation
+(define has-reactive-reset-frame?
+  (fn (kont)
+    (if (empty? kont) false
+      (if (= (frame-type (first kont)) "reactive-reset") true
+        (has-reactive-reset-frame? (rest kont))))))
+
+;; Capture frames up to nearest ReactiveResetFrame.
+;; Returns (captured-frames, reset-frame, remaining-kont).
+(define kont-capture-to-reactive-reset
+  (fn (kont)
+    (define scan
+      (fn (k captured)
+        (if (empty? k)
+          (error "reactive deref without enclosing reactive-reset")
+          (let ((frame (first k)))
+            (if (= (frame-type frame) "reactive-reset")
+              (list captured frame (rest k))
+              (scan (rest k) (append captured (list frame))))))))
+    (scan kont (list))))