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/ref/__init__.py

@@ -0,0 +1,7 @@
+"""Reference SX evaluator — transpiled from canonical .sx spec files.
+
+This package provides the bootstrap-compiled evaluator as an alternative
+backend to the hand-written evaluator.py / html.py / async_eval.py.
+
+Enable by setting SX_USE_REF=1 environment variable.
+"""