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

@@ -26,6 +26,7 @@ DEPTH       = 2
 N_HEADS     = 4
 ALPHA       = 0.1   # VICReg mixing weight (fixed at 0.1 in HEPA paper)
 DELTA_T_MAX = 3     # max prediction horizon in patches (1..min(DELTA_T_MAX, N-1-c))
+BATCH_SIZE  = 512   # mini-batch per step (hourly dataset is too large for full-batch)
 EPOCHS      = 300
 LR          = 3e-4
 SEED        = 0
@@ -157,29 +158,37 @@ def main():
     (Xtr, ytr), (Xte, yte) = build()
     n_feats   = Xtr.shape[2]
     n_patches = WINDOW // PATCH_LEN
-    Xtr_t     = torch.tensor(Xtr, device=dev)
+    N_tr      = len(Xtr)
+    bs        = min(BATCH_SIZE, N_tr)
 
     enc  = CausalEncoder(n_feats, PATCH_LEN, D_MODEL, N_HEADS, DEPTH).to(dev)
     pred = HorizonPredictor(D_MODEL).to(dev)
     opt  = torch.optim.AdamW(list(enc.parameters()) + list(pred.parameters()), lr=LR)
 
     for ep in range(EPOCHS):
-        # Sample random context position and horizon; Δt log-biased toward short
+        # Random mini-batch (avoids OOM on large hourly dataset)
+        idx_b = torch.randperm(N_tr)[:bs]
+        Xb    = torch.tensor(Xtr[idx_b.numpy()], device=dev)
+
+        # Sample random context position and horizon
         c  = torch.randint(0, n_patches - 1, ()).item()
         dt = torch.randint(1, max(2, min(DELTA_T_MAX, n_patches - 1 - c) + 1), ()).item()
 
-        tokens = enc(Xtr_t)                     # (B, N, D)
+        tokens = enc(Xb)                        # (bs, N, D)
         h_ctx  = tokens[:, c, :]               # context embedding
         h_tgt  = tokens[:, c + dt, :]          # target embedding (joint training)
-        h_hat  = pred(h_ctx, torch.full((len(Xtr),), float(dt), device=dev))
+        h_hat  = pred(h_ctx, torch.full((bs,), float(dt), device=dev))
         loss   = vicreg_loss(h_hat, h_tgt, alpha=ALPHA)
         opt.zero_grad(); loss.backward(); opt.step()
 
     enc.eval()
     with torch.no_grad():
         def embed(X_np):
-            t = torch.tensor(X_np, device=dev)
-            return enc(t)[:, -1, :].cpu().numpy()   # last token = full-context summary
+            chunks = []
+            for i in range(0, len(X_np), bs):
+                t = torch.tensor(X_np[i:i+bs], device=dev)
+                chunks.append(enc(t)[:, -1, :].cpu().numpy())
+            return np.concatenate(chunks, axis=0)
 
         Etr = embed(Xtr)
         Ete = embed(Xte)
@@ -223,14 +232,18 @@ 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)
+            chunks = []
             with torch.no_grad():
-                E = enc(torch.tensor(np.stack(Xs), device=dev))[:, -1, :].cpu().numpy().tolist()
+                for i in range(0, len(Xa), bs):
+                    chunks.append(enc(torch.tensor(Xa[i:i+bs], device=dev))[:, -1, :].cpu().numpy())
+            E = np.concatenate(chunks, axis=0).tolist()
             return E, dates, rvs
         Etr2, dates_tr, rv_tr = _export_windows(tr_mask)
         Eoos, dates_oos, rv_oos = _export_windows(df2["date"].dt.year >= 2022)