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-compiler.sx

@@ -292,7 +292,7 @@
             (let
               ((obj (hs-to-sx (nth expr 1))) (prop (nth expr 2)))
               (list
-                (quote host-set)
+                (quote host-set!)
                 obj
                 prop
                 (list
@@ -344,7 +344,7 @@
             (let
               ((obj (hs-to-sx (nth expr 1))) (prop (nth expr 2)))
               (list
-                (quote host-set)
+                (quote host-set!)
                 obj
                 prop
                 (list
@@ -815,6 +815,18 @@
                       (nth ast 1)))))
               ((= head (quote remove-element))
                 (list (quote dom-remove) (hs-to-sx (nth ast 1))))
+              ((= head (quote empty-target))
+                (list (quote hs-empty-target!) (hs-to-sx (nth ast 1))))
+              ((= head (quote swap!))
+                (let
+                  ((lhs (nth ast 1)) (rhs (nth ast 2)))
+                  (list
+                    (quote let)
+                    (list (list (quote _swap_tmp) (hs-to-sx lhs)))
+                    (list
+                      (quote do)
+                      (emit-set lhs (hs-to-sx rhs))
+                      (emit-set rhs (quote _swap_tmp))))))
               ((= head (quote remove-attr))
                 (let
                   ((tgt (if (nil? (nth ast 2)) (quote me) (hs-to-sx (nth ast 2)))))