Document S/R findings and isolate legacy gate features

scripts/run_backtest_snapshot.py

@@ -53,6 +53,8 @@ def _parse_args() -> argparse.Namespace:
             "soft_zones", "confirmed_rounds", "gate_v2",
             "rewrite_legacy_primary", "soft_zones_legacy_primary",
             "confirmed_rounds_legacy_primary", "gate_v2_legacy_primary",
+            "legacy_geometry_neutral", "legacy_pivots_only",
+            "legacy_traffic_grid_only",
         ),
         default=None,
         help="Research-only S/R detector/gate arm.",