Wire SX_USE_REF=1 switching through all route intermediaries

jinja_bridge, helpers, handlers, query_executor now conditionally
import from ref/sx_ref and ref/async_eval_ref when SX_USE_REF=1.

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

shared/sx/handlers.py

@@ -69,7 +69,11 @@ def clear_handlers(service: str | None = None) -> None:
 def load_handler_file(filepath: str, service_name: str) -> list[HandlerDef]:
     """Parse an .sx file, evaluate it, and register any HandlerDef values."""
     from .parser import parse_all
-    from .evaluator import _eval as _raw_eval, _trampoline
+    import os
+    if os.environ.get("SX_USE_REF") == "1":
+        from .ref.sx_ref import eval_expr as _raw_eval, trampoline as _trampoline
+    else:
+        from .evaluator import _eval as _raw_eval, _trampoline
     _eval = lambda expr, env: _trampoline(_raw_eval(expr, env))
     from .jinja_bridge import get_component_env
 
@@ -126,7 +130,11 @@ async def execute_handler(
     4. Return ``SxExpr`` wire format
     """
     from .jinja_bridge import get_component_env, _get_request_context
-    from .async_eval import async_eval_to_sx
+    import os
+    if os.environ.get("SX_USE_REF") == "1":
+        from .ref.async_eval_ref import async_eval_to_sx
+    else:
+        from .async_eval import async_eval_to_sx
     from .types import NIL
 
     if args is None: