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/bootstrap_js.py

@@ -2776,6 +2776,7 @@ PLATFORM_DOM_JS = """
 
   // Register DOM adapter as the render dispatch target for the evaluator.
   _renderExprFn = function(expr, env) { return renderToDom(expr, env, null); };
+  _renderMode = true;  // Browser always evaluates in render context.
 
   var SVG_NS = "http://www.w3.org/2000/svg";
   var MATH_NS = "http://www.w3.org/1998/Math/MathML";