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-parser.sx

@@ -1422,6 +1422,21 @@
             (let
               ((end-pos (skip-to-close 0)))
               (substring src start-pos end-pos)))))
+      (define
+        parse-empty-cmd
+        (fn
+          ()
+          (let
+            ((target (cond ((at-end?) (list (quote sym) "me")) ((and (= (tp-type) "keyword") (or (= (tp-val) "then") (= (tp-val) "end"))) (list (quote sym) "me")) (true (parse-expr)))))
+            (list (quote empty-target) target))))
+      (define
+        parse-swap-cmd
+        (fn
+          ()
+          (let
+            ((lhs (parse-expr)))
+            (match-kw "with")
+            (let ((rhs (parse-expr))) (list (quote swap!) lhs rhs)))))
       (define
         parse-cmd
         (fn
@@ -1503,6 +1518,12 @@
                 (do (adv!) (parse-halt-cmd)))
               ((and (= typ "keyword") (= val "focus"))
                 (do (adv!) (parse-focus-cmd)))
+              ((and (= typ "keyword") (= val "empty"))
+                (do (adv!) (parse-empty-cmd)))
+              ((and (= typ "keyword") (= val "clear"))
+                (do (adv!) (parse-empty-cmd)))
+              ((and (= typ "keyword") (= val "swap"))
+                (do (adv!) (parse-swap-cmd)))
               (true (parse-expr))))))
       (define
         parse-cmd-list
@@ -1548,7 +1569,10 @@
                 (= v "scroll")
                 (= v "select")
                 (= v "reset")
-                (= v "focus"))))
+                (= v "focus")
+                (= v "empty")
+                (= v "clear")
+                (= v "swap"))))
           (define
             cl-collect
             (fn