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>

web/adapter-dom.sx

@@ -342,7 +342,7 @@
                         (= attr-name "ref")
                         (let
                           ((attr-val (trampoline (eval-expr attr-expr env))))
-                          (dict-set! attr-val "current" el))
+                          (if (callable? attr-val) (attr-val el) (dict-set! attr-val "current" el)))
                         (= attr-name "key")
                         (let
                           ((attr-val (trampoline (eval-expr attr-expr env))))