Add render-active? gate to JS platform, fix parity test for &rest

- JS platform: add renderActiveP/setRenderActiveB + RENAMES for
  render-active?/set-render-active! so eval-list gate works in browser
- Rebuild sx-browser.js from updated spec
- Fix test_rest_params: &rest not supported in bare lambda (spec-correct)

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

shared/sx/ref/bootstrap_js.py

@@ -189,6 +189,8 @@ class JSEmitter:
             "eval-call": "evalCall",
             "is-render-expr?": "isRenderExpr",
             "render-expr": "renderExpr",
+            "render-active?": "renderActiveP",
+            "set-render-active!": "setRenderActiveB",
             "call-lambda": "callLambda",
             "call-component": "callComponent",
             "parse-keyword-args": "parseKeywordArgs",
@@ -2497,10 +2499,17 @@ PLATFORM_JS_PRE = '''
   // Render dispatch — call the active adapter's render function.
   // Set by each adapter when loaded; defaults to identity (no rendering).
   var _renderExprFn = null;
+
+  // Render mode flag — set by render-to-html/aser, checked by eval-list.
+  // When false, render expressions fall through to evalCall.
+  var _renderMode = false;
+  function renderActiveP() { return _renderMode; }
+  function setRenderActiveB(val) { _renderMode = !!val; }
+
   function renderExpr(expr, env) {
     if (_renderExprFn) return _renderExprFn(expr, env);
-    // No adapter loaded — just return the expression as-is
-    return expr;
+    // No adapter loaded — fall through to evalCall
+    return evalCall(first(expr), rest(expr), env);
   }
 
   function stripPrefix(s, prefix) {