Fix win_rate() for Kalman Pairs: add net_pnl/gross_pnl field support

Bug: win_rate() only checked pnl_net/pnl_gross/pnl fields,
but Kalman backtests save trades with net_pnl/gross_pnl (underscore-first).
Result: all 4 Kalman assets showed 0% win on 27-35 trades.

After fix:
  BTC: 0% → 45% (16/35)
  ETH: 0% → 47% (16/34)
  HYPE: 0% → 51% (14/27)
  VVV: 0% → 57% (19/33)

Also corrected paper trader coin assignments for Mean Reversion
and Momentum Breakout (was BTC, should be ETH).

common/metrics.py

@@ -40,5 +40,7 @@ def max_drawdown(equity: list[float]) -> float:
 def win_rate(trades: list[dict]) -> float:
     if not trades:
         return 0.0
-    tp = sum(1 for t in trades if (t.get("pnl_net") or t.get("pnl_gross") or t.get("pnl", 0)) > 0)
+    tp = sum(1 for t in trades if (
+        (t.get("pnl_net") or t.get("net_pnl") or t.get("pnl_gross") or t.get("gross_pnl") or t.get("pnl", 0)) > 0
+    ))
     return tp / len(trades)