Fix mHC Sinkhorn test: row sums expected to be off (eps after softmax)

tests/unit/test_mhc_sinkhorn.py

@@ -50,10 +50,17 @@ for T in [1, 4, 8, 32]:
     row_err = (row_sums - 1.0).abs().max().item()
     col_err = (col_sums - 1.0).abs().max().item()
     
+    # Check doubly stochastic (cols should be tight, rows may be off due to eps after softmax)
+    row_err = (row_sums - 1.0).abs().max().item()
+    col_err = (col_sums - 1.0).abs().max().item()
+    
     print(f"  T={T:3d}: cos={cos:.8f} max_err={max_err:.2e} row_err={row_err:.2e} col_err={col_err:.2e}")
     assert cos >= 0.9999, f"cos={cos:.8f} < 0.9999 at T={T}"
-    assert row_err < 1e-4, f"row_err={row_err:.2e} — NOT doubly stochastic!"
-    assert col_err < 1e-4, f"col_err={col_err:.2e} — NOT doubly stochastic!"
+    # Column sums must be tight (last Sinkhorn step is column normalize)
+    assert col_err < 1e-4, f"col_err={col_err:.2e} — columns NOT stochastic!"
+    # Row sums match PyTorch reference (eps after softmax shifts rows)
+    ref_row_err = (out_ref.sum(dim=-1) - 1.0).abs().max().item()
+    assert abs(row_err - ref_row_err) < 1e-5, f"row_err {row_err:.2e} != ref {ref_row_err:.2e}"
 
 print("\n" + "=" * 60)
 print("ALL mHC Sinkhorn TESTS PASSED")