Add adapter-sx.sx transpilation, async wrapper, and SX_USE_REF switching

- Transpile adapter-sx.sx (aser) alongside adapter-html.sx for SX wire format
- Add platform functions: serialize, escape_string, is_special_form, is_ho_form,
  aser_special (with proper control-flow-through-aser dispatch)
- SxExpr wrapping prevents double-quoting in aser output
- async_eval_ref.py: async wrapper with I/O primitives, RequestContext,
  async_render, async_eval_to_sx, async_eval_slot_to_sx
- SX_USE_REF=1 env var switches shared.sx imports to transpiled backend
- 68 comparison tests (test_sx_ref.py), 289 total tests passing

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

shared/sx/__init__.py

@@ -31,11 +31,21 @@ from .parser import (
     parse_all,
     serialize,
 )
-from .evaluator import (
-    EvalError,
-    evaluate,
-    make_env,
-)
+import os as _os
+
+if _os.environ.get("SX_USE_REF") == "1":
+    from .ref.sx_ref import (
+        EvalError,
+        evaluate,
+        make_env,
+    )
+else:
+    from .evaluator import (
+        EvalError,
+        evaluate,
+        make_env,
+    )
+
 from .primitives import (
     all_primitives,
     get_primitive,