Fix: NameError 'sym' undefined in parking overlay gate check

Should be actual_sym (defined in _parking_evaluate_gate scope),
not sym (which is only defined in _parking_buy).

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

apps/paper_trader/engine.py

@@ -1345,7 +1345,7 @@ class PaperTradingEngine:
 
         # Low-vol overlay: upgrade base symbol (e.g. QQQM → TQQQ) when market is calm
         overlay_sym = (risk.cash_parking_low_vol_overlay_symbol or "").upper()
-        if overlay_sym and overlay_sym != sym and self._parking_brake_cooldown_remaining <= 0:
+        if overlay_sym and overlay_sym != actual_sym.upper() and self._parking_brake_cooldown_remaining <= 0:
 
             def _compute_hurst(lb: int) -> float | None:
                 if n < lb + 1: