fix: define zero-column rank detection contract

diff_diff/linalg.py

@@ -134,7 +134,9 @@ def _detect_rank_deficiency(
     pivot : ndarray of int
         Column permutation from QR decomposition.
     """
-    n, k = X.shape
+    _, k = X.shape
+    if k == 0:
+        return 0, np.array([], dtype=int), np.array([], dtype=int)
 
     # R's qr() uses tol = 1e-07 by default (sqrt(eps) ≈ 1.49e-08); we use 1e-07.
     if rcond is None:

tests/test_linalg.py

@@ -1525,6 +1525,20 @@ def test_solve_ols_rank_zero_returns_nan_not_indexerror(self):
         np.testing.assert_allclose(resid, y)  # fitted = 0
         assert vcov.shape == (3, 3) and np.all(np.isnan(vcov))
 
+    def test_rank_detection_zero_column_matrix_returns_empty_contract(self):
+        """An ``(n, 0)`` design has rank 0, no dropped columns, and no pivot."""
+        from diff_diff.linalg import _detect_rank_deficiency
+
+        X = np.empty((5, 0))
+
+        rank, dropped, pivot = _detect_rank_deficiency(X)
+
+        assert rank == 0
+        assert dropped.dtype == int
+        assert pivot.dtype == int
+        assert dropped.shape == (0,)
+        assert pivot.shape == (0,)
+
     def test_rank_detection_scale_repair_preserves_raw_drop_selection(self):
         """The scale-invariance repair must NOT change which column is dropped in a
         genuinely collinear, well-scaled design: the dropped column equals the raw