docs: show verified diagnostic and result output in thesis walkthrough

tests/examples/test_thesis_python_walkthrough.py

@@ -73,4 +73,14 @@ async def test_thesis_python_session_preserves_report_and_repair(
     diagnostic = capsys.readouterr().out
     assert "invalid_source_path" in diagnostic
     assert "output[0].path" in diagnostic
+    # Marked text blocks are verbatim stdout excerpts, checked against the real
+    # session so displayed diagnostics/results cannot drift from execution.
+    displayed_outputs = re.findall(
+        r"^<!-- walkthrough-output -->\n```text\n(.*?)^```",
+        chapter,
+        re.MULTILINE | re.DOTALL,
+    )
+    assert len(displayed_outputs) == 2, "Retain the diagnostic and result excerpts"
+    for displayed in displayed_outputs:
+        assert displayed in diagnostic, "Displayed output differs from the session"
     assert namespace["trace"].frames