Fix browser: set render-active in DOM adapter + on adapter init

Browser always evaluates in render context — _renderMode must be true
when DOM adapter is loaded, and render-to-dom must call set-render-active!.
Fixes 'Undefined symbol: <>' error in browser.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/adapter-dom.sx

@@ -20,6 +20,7 @@
 
 (define render-to-dom
   (fn (expr env ns)
+    (set-render-active! true)
     (case (type-of expr)
       ;; nil / boolean false / boolean true → empty fragment
       "nil"       (create-fragment)