Isolate explicit S/R gate target ladder

scripts/run_backtest_snapshot.py

@@ -60,6 +60,7 @@ def _parse_args() -> argparse.Namespace:
             "rewrite_range504_structural_legacy_primary",
             "rewrite_range504_structural_primary2",
             "production_structural_overlay",
+            "explicit_target_ladder",
         ),
         default=None,
         help="Research-only S/R detector/gate arm.",