fix: surface BusinessReport appendix render failures

diff_diff/business_report.py

@@ -353,12 +353,21 @@ def full_report(self) -> str:
         """Return a structured multi-section markdown report."""
         base = _render_full_report(self.to_dict())
         if self._include_appendix:
+            appendix_text = None
             try:
                 appendix = self._results.summary()
-            except Exception:  # noqa: BLE001
-                appendix = None
-            if appendix:
-                base = base + "\n\n## Technical Appendix\n\n```\n" + str(appendix) + "\n```\n"
+                if appendix:
+                    appendix_text = str(appendix)
+            except Exception as exc:  # noqa: BLE001
+                appendix_error = type(exc).__name__ or "Exception"
+                base = (
+                    base
+                    + "\n\n## Technical Appendix\n\n"
+                    + "Technical appendix unavailable: estimator summary rendering failed "
+                    + f"({appendix_error}).\n"
+                )
+            if appendix_text:
+                base = base + "\n\n## Technical Appendix\n\n```\n" + appendix_text + "\n```\n"
         return base
 
     def export_markdown(self) -> str:

tests/test_business_report.py

@@ -480,6 +480,40 @@ def test_include_appendix_false_omits(self, event_study_fit):
         md = br.full_report()
         assert "## Technical Appendix" not in md
 
+    def test_appendix_summary_failure_is_visible(self, event_study_fit):
+        fit, _ = event_study_fit
+        with patch.object(fit, "summary", side_effect=RuntimeError("internal path /tmp/private")):
+            br = BusinessReport(fit, auto_diagnostics=False, include_appendix=True)
+            md = br.full_report()
+
+        assert "## Technical Appendix" in md
+        assert "Technical appendix unavailable" in md
+        assert "RuntimeError" in md
+
+    def test_appendix_summary_failure_does_not_leak_exception_details(self, event_study_fit):
+        fit, _ = event_study_fit
+        with patch.object(fit, "summary", side_effect=RuntimeError("secret-token-123")):
+            br = BusinessReport(fit, auto_diagnostics=False, include_appendix=True)
+            md = br.full_report()
+
+        assert "secret-token-123" not in md
+        assert "Traceback" not in md
+
+    def test_appendix_stringification_failure_is_visible(self, event_study_fit):
+        class _BadSummary:
+            def __str__(self):
+                raise ValueError("sensitive-render-context")
+
+        fit, _ = event_study_fit
+        with patch.object(fit, "summary", return_value=_BadSummary()):
+            br = BusinessReport(fit, auto_diagnostics=False, include_appendix=True)
+            md = br.full_report()
+
+        assert "## Technical Appendix" in md
+        assert "Technical appendix unavailable" in md
+        assert "ValueError" in md
+        assert "sensitive-render-context" not in md
+
 
 # ---------------------------------------------------------------------------
 # BaconDecompositionResults