Delete evaluator.py shim: all imports go directly to bootstrapped sx_ref.py

EvalError moved to types.py. All 27 files updated to import eval_expr,
trampoline, call_lambda, etc. directly from shared.sx.ref.sx_ref instead
of through the evaluator.py indirection layer. 320/320 spec tests pass.

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

shared/sx/__init__.py

@@ -31,11 +31,8 @@ from .parser import (
     parse_all,
     serialize,
 )
-from .evaluator import (
-    EvalError,
-    evaluate,
-    make_env,
-)
+from .types import EvalError
+from .ref.sx_ref import evaluate, make_env
 
 from .primitives import (
     all_primitives,