Fix double-escaping when render forms (<>, HTML tags) appear in eval position

Return _RawHTML wrapper so pre-rendered HTML in let bindings isn't
escaped when used in render context.

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

shared/sx/async_eval.py

@@ -102,9 +102,11 @@ async def async_eval(expr: Any, env: dict[str, Any], ctx: RequestContext) -> Any
                 return await async_eval(expanded, env, ctx)
 
         # Render forms in eval position — delegate to renderer and return
-        # the HTML string.  Allows (let ((x (<> ...))) ...) etc.
+        # as _RawHTML so it won't be double-escaped when used in render
+        # context later.  Allows (let ((x (<> ...))) ...) etc.
         if name in ("<>", "raw!") or name in HTML_TAGS:
-            return await _arender(expr, env, ctx)
+            html = await _arender(expr, env, ctx)
+            return _RawHTML(html)
 
     # --- function / lambda call ---
     fn = await async_eval(head, env, ctx)