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/tests/run.py

@@ -771,6 +771,15 @@ def main():
             print(f"# --- {spec_name} ---")
             eval_file(spec["file"], env)
 
+            # Reset render state after render tests to avoid leaking
+            # into subsequent specs (bootstrapped evaluator checks render_active)
+            if spec_name == "render":
+                try:
+                    from shared.sx.ref.sx_ref import set_render_active_b
+                    set_render_active_b(False)
+                except ImportError:
+                    pass
+
     # Summary
     print()
     print(f"1..{test_num}")