Replace hand-written evaluator with bootstrapped spec, emit flat Python

- evaluator.py: replace 1200 lines of hand-written eval with thin shim
  that re-exports from bootstrapped sx_ref.py
- bootstrap_py.py: emit all fn-bodied defines as `def` (not `lambda`),
  flatten tail-position if/cond/case/when to if/elif with returns,
  fix &rest handling in _emit_define_as_def
- platform_py.py: EvalError imports from evaluator.py so catches work
- __init__.py: remove SX_USE_REF conditional, always use bootstrapped
- tests/run.py: reset render_active after render tests for isolation
- Removes setrecursionlimit(5000) hack — no longer needed with flat code

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

shared/sx/__init__.py

@@ -31,20 +31,11 @@ from .parser import (
     parse_all,
     serialize,
 )
-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 .evaluator import (
+    EvalError,
+    evaluate,
+    make_env,
+)
 
 from .primitives import (
     all_primitives,