fix(eval): export block uses next-period RV target (t+1) to match Python probe

Go harness reported 0.42 vs Python 0.36 because export used realized_vol[t]
(current) while Python probe used realized_vol[t+1] (next-period). Fix adds
t+1 < len(df2) guard and uses iloc[t+1] as target. Go now matches Python: 0.3585.

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

train.py

@@ -232,10 +232,10 @@ def main():
             idx = df2.index[year_mask].tolist()
             Xs, dates, rvs = [], [], []
             for t in idx:
-                if t - WINDOW >= 0:
+                if t - WINDOW >= 0 and t + 1 < len(df2):
                     Xs.append(fn2[t - WINDOW:t])
                     dates.append(str(df2["date"].iloc[t].date()))
-                    rvs.append(float(df2["realized_vol"].iloc[t]))
+                    rvs.append(float(df2["realized_vol"].iloc[t + 1]))
             if not Xs:
                 return [], [], []
             Xa = np.stack(Xs)