Fix :ref callback bug in adapter-dom — Pretext island fully working

Root cause: adapter-dom.sx line 345 handled :ref by calling
(dict-set! attr-val "current" el), assuming React-style ref objects.
Callback-style refs (fn (el) ...) passed a function, not a dict,
causing dict-set! to fail with "dict key val" error.

Fix: (if (callable? attr-val) (attr-val el) (dict-set! attr-val "current" el))
Supports both callback refs and dict refs.

Pretext island now fully working:
- 3 controls: width slider, font size slider, algorithm toggle
- Knuth-Plass + greedy line breaking via bytecode-compiled library
- canvas.measureText for pixel-perfect browser font metrics
- Effect-based imperative DOM rendering (createElement + appendChild)
- Reactive: slider drag → re-measure → re-break → re-render

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

shared/static/wasm/sx/hs-runtime.sx

@@ -444,10 +444,31 @@
       ((dict? v) (= (len (keys v)) 0))
       (true false))))
 ;; Array slicing (inclusive both ends)
-(define hs-first (fn (lst) (first lst)))
+(define
+  hs-empty-target!
+  (fn
+    (target)
+    (cond
+      ((list? target) (for-each (fn (el) (hs-empty-target! el)) target))
+      ((nil? target) nil)
+      (true
+        (let
+          ((tag (dom-get-prop target "tagName")))
+          (cond
+            ((or (= tag "INPUT") (= tag "TEXTAREA"))
+              (let
+                ((input-type (dom-get-prop target "type")))
+                (if
+                  (or (= input-type "checkbox") (= input-type "radio"))
+                  (dom-set-prop target "checked" false)
+                  (dom-set-prop target "value" ""))))
+            ((= tag "FORM") (dom-set-inner-html target ""))
+            (true (dom-set-inner-html target ""))))))))
 ;; Collection: sorted by
-(define hs-last (fn (lst) (last lst)))
+(define hs-first (fn (lst) (first lst)))
 ;; Collection: sorted by descending
+(define hs-last (fn (lst) (last lst)))
+;; Collection: split by
 (define
   hs-template
   (fn
@@ -533,7 +554,7 @@
                   (set! i (+ i 1))
                   (tpl-loop)))))))
       (do (tpl-loop) result))))
-;; Collection: split by
+;; Collection: joined by
 (define
   hs-make-object
   (fn
@@ -545,7 +566,7 @@
           (fn (pair) (dict-set! d (first pair) (nth pair 1)))
           pairs)
         d))))
-;; Collection: joined by
+
 (define
   hs-method-call
   (fn