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/types.py

@@ -375,6 +375,15 @@ class _ShiftSignal(BaseException):
         self.env = env
 
 
+# ---------------------------------------------------------------------------
+# EvalError
+# ---------------------------------------------------------------------------
+
+class EvalError(Exception):
+    """Error during expression evaluation."""
+    pass
+
+
 # ---------------------------------------------------------------------------
 # Type alias
 # ---------------------------------------------------------------------------