Address PR #401 R4 review (1 P3 maintainability)

R4 verdict was "Looks good". One P3 housekeeping item addressed:

P3 (maintainability): the `summary()` empty-state renderer for
`path_effects == {}` rendered the same generic "No observed paths
have a complete [F_g-1, F_g-1+L_max] window" text under both
selectors. Under `paths_of_interest`, that text is misleading
because the panel may have observed paths but none match the user's
list. Branch on `_estimator_ref.paths_of_interest`: when set, the
empty-state block now reads "Every path in paths_of_interest was
unobserved or had a window outside L_max+1" with a pointer to the
per-path 'zero observed groups' UserWarnings; under `by_path=k`,
the original wording is preserved.

Adds regression
`test_paths_of_interest_all_unobserved_summary_distinct_text` (next
to the existing `_emits_distinct_warning` test) asserting the
selector-specific summary text fires and the by_path-only wording
does not.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/chaisemartin_dhaultfoeuille_results.py

@@ -1267,14 +1267,26 @@ def _render_path_effects_section(
         if self.path_effects is None:
             return
         if not self.path_effects:
+            # Distinguish the two empty causes for paths_of_interest
+            # users (every requested path unobserved) from by_path=k
+            # users (no panel path has a complete window).
+            poi = getattr(self._estimator_ref, "paths_of_interest", None)
+            if poi is not None:
+                detail_lines = [
+                    "  Every path in paths_of_interest was unobserved or had a window outside L_max+1.",
+                    "  (See per-path 'zero observed groups' UserWarnings emitted at fit().)",
+                ]
+            else:
+                detail_lines = [
+                    "  No observed paths have a complete [F_g-1, F_g-1+L_max] window.",
+                    "  (See UserWarning emitted at fit(); by_path was a no-op on this panel.)",
+                ]
             lines.extend(
                 [
                     thin,
                     "Treatment-Path Disaggregation".center(width),
                     thin,
-                    "  No observed paths have a complete [F_g-1, F_g-1+L_max] window.",
-                    "  (See UserWarning emitted at fit(); by_path / "
-                    "paths_of_interest was a no-op on this panel.)",
+                    *detail_lines,
                     thin,
                     "",
                 ]

tests/test_chaisemartin_dhaultfoeuille.py

@@ -8732,6 +8732,33 @@ def test_paths_of_interest_frequency_rank_is_true_frequency(self):
         assert res.path_effects[(0, 1, 1, 1)]["frequency_rank"] == 1
         assert res.path_effects[(0, 1, 0, 0)]["frequency_rank"] == 2
 
+    def test_paths_of_interest_all_unobserved_summary_distinct_text(self):
+        """When every path in `paths_of_interest` is unobserved, the
+        empty-state `summary()` block must render the
+        paths_of_interest-specific text rather than the generic
+        "no observed paths have a complete window" text (regression
+        for R4 P3 maintainability finding)."""
+        df = _by_path_three_path_data()
+        est = ChaisemartinDHaultfoeuille(
+            drop_larger_lower=False,
+            paths_of_interest=[(1, 1, 1, 1), (1, 0, 1, 0)],  # both unobserved
+            twfe_diagnostic=False,
+            seed=42,
+        )
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", UserWarning)
+            res = est.fit(
+                df, outcome="outcome", group="group", time="period",
+                treatment="treatment", L_max=3,
+            )
+        text = res.summary()
+        assert "Every path in paths_of_interest was unobserved" in text, (
+            f"summary() did not render the paths_of_interest-specific "
+            f"empty-state text. Got:\n{text}"
+        )
+        # And the generic by_path-only wording must NOT appear in this case.
+        assert "No observed paths have a complete" not in text
+
     def test_paths_of_interest_all_unobserved_emits_distinct_warning(self):
         """When every path in `paths_of_interest` is unobserved,
         the empty-state warning should mention `paths_of_interest`