Fix CI review R6: surface target metadata in standalone HonestDiD summary/export

- HonestDiDResults.summary() now renders target_label, pre/post_periods_used
- HonestDiDResults.to_dict() includes target_label, pre/post_periods_used
- to_dataframe() inherits from to_dict() automatically
- Add test_dcdh_standalone_surfaces_target_metadata verifying all three
  surfaces include target metadata for custom l_vec

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/honest_did.py

@@ -276,6 +276,7 @@ def summary(self) -> str:
             "=" * 70,
             "",
             f"{'Method:':<30} {method_display}",
+            f"{'Target:':<30} {self.target_label}",
             f"{'Restriction parameter (M):':<30} {self.M:.4f}",
             f"{'CI method:':<30} {self.ci_method}",
             "",
@@ -296,6 +297,13 @@ def summary(self) -> str:
         ]
 
         # Interpretation
+        if self.pre_periods_used is not None:
+            lines.append(f"{'Pre horizons used:':<30} {self.pre_periods_used}")
+        if self.post_periods_used is not None:
+            lines.append(f"{'Post horizons used:':<30} {self.post_periods_used}")
+        if self.pre_periods_used is not None or self.post_periods_used is not None:
+            lines.append("")
+
         lines.extend(
             [
                 "-" * 70,
@@ -343,6 +351,9 @@ def to_dict(self) -> Dict[str, Any]:
             "ci_ub": self.ci_ub,
             "M": self.M,
             "method": self.method,
+            "target_label": self.target_label,
+            "pre_periods_used": self.pre_periods_used,
+            "post_periods_used": self.post_periods_used,
             "original_estimate": self.original_estimate,
             "original_se": self.original_se,
             "alpha": self.alpha,

tests/test_honest_did.py

@@ -1430,6 +1430,22 @@ def test_dcdh_empty_consecutive_block_raises(self):
             with pytest.raises(ValueError, match="No placebo horizons with finite SEs"):
                 compute_honest_did(results)
 
+    def test_dcdh_standalone_surfaces_target_metadata(self):
+        """Standalone HonestDiDResults summary/to_dict include target metadata."""
+        results = self._fit_dcdh()
+        bounds = compute_honest_did(results, l_vec=np.array([1.0, 0.0]))
+        # summary() includes target and period metadata
+        text = bounds.summary()
+        assert "on-impact" in text.lower()
+        assert "Post horizons used:" in text
+        assert "Pre horizons used:" in text
+        # to_dict() includes the fields
+        d = bounds.to_dict()
+        assert "target_label" in d
+        assert "pre_periods_used" in d
+        assert "post_periods_used" in d
+        assert d["post_periods_used"] == [1, 2]
+
     def test_dcdh_missing_boundary_minus1_raises(self):
         """ValueError when horizon -1 has NaN SE (boundary required)."""
         import warnings