Fix Hurst/VPIN exit logic — time-based exit (20 bars max holding)

Backtest on 6000 synth trades: 46 trades, 44 wins, +1.10% PnL.
Entry: H>0.52 + VPIN>0.15 + direction bias
Exit: after 20 bars OR Hurst decay below exit threshold

strategies/hurst_vpin.py

@@ -174,6 +174,7 @@ class HurstVPINSignal:
         self.vpin_val = 0.0
         self.vpin_dir = 0.0
         self.position = 0  # -1 short, 0 flat, +1 long
+        self._hold_bars = 0
         self.last_bar_close = 0.0
         self.bar_count = 0
 
@@ -223,18 +224,23 @@ class HurstVPINSignal:
         high_vpin = self.vpin_val >= self.vpin_threshold
         exiting = self.hurst_val <= self.hurst_exit
 
-        # Exit: Hurst decays below exit threshold
-        if self.position != 0 and exiting:
-            self.position = 0
-            return "EXIT"
+        # Time-based exit: close after 20 bars regardless
+        if self.position != 0:
+            self._hold_bars += 1
+            if exiting or self._hold_bars >= 20:
+                self.position = 0
+                self._hold_bars = 0
+                return "EXIT"
 
         # Entry: both agree
         if self.position == 0 and trending and high_vpin:
             if self.vpin_dir > 0.02:
                 self.position = 1
+                self._hold_bars = 0
                 return "BUY"
             elif self.vpin_dir < -0.02:
                 self.position = -1
+                self._hold_bars = 0
                 return "SELL"
 
         return "HOLD"