fix(notebooks): drop cascade errors from snapshot parsing

marimo marks every descendant of a failed or stopped cell with its own
error output ('An ancestor raised an exception', 'ancestor-stopped').
The root cause is always present as a non-ancestor error in the same
snapshot, so cascades only inflate the report — and each one filed its
own deduplicated issue (6 of the 12 nb issues from the first nightly
run were cascade noise). Also makes intentional mo.stop() flow control
count as a pass instead of a failure.

dev/scripts/nb_integration.py

@@ -82,6 +82,14 @@ def parse_snapshot(path: Path) -> list[dict]:
         for out in cell.get("outputs", []):
             if out.get("type") != "error":
                 continue
+            # Cascade noise: marimo marks every descendant of a failed or
+            # stopped cell with its own error output. The root cause is
+            # always present as a non-ancestor error in the same snapshot,
+            # so these only inflate the report (and the issue tracker).
+            if out.get("ename") == "ancestor-stopped" or out.get(
+                "evalue", ""
+            ).startswith("An ancestor raised an exception"):
+                continue
             detail = ""
             for con in cell.get("console", []):
                 if con.get("name") == "stderr":

tests/dev/test_nb_integration.py

@@ -86,6 +86,45 @@ def test_parse_snapshot_extracts_errors_with_console_detail(tmp_path):
     assert "intentional failure" in e["detail"]
 
 
+def test_parse_snapshot_drops_cascade_errors(tmp_path):
+    """Descendants of a failed/stopped cell get their own error outputs;
+    only the root-cause error should survive parsing."""
+    snap = {
+        "version": "1",
+        "metadata": {"marimo_version": "0.23.13"},
+        "cells": [
+            SNAPSHOT_ERR["cells"][1],
+            {
+                "id": "ecfG",
+                "outputs": [
+                    {
+                        "type": "error",
+                        "ename": "exception",
+                        "evalue": "An ancestor raised an exception (ValueError): ",
+                        "traceback": None,
+                    }
+                ],
+                "console": [],
+            },
+            {
+                "id": "xXTn",
+                "outputs": [
+                    {
+                        "type": "error",
+                        "ename": "ancestor-stopped",
+                        "evalue": "This cell wasn't run because an ancestor "
+                        "was stopped with `mo.stop`: ",
+                        "traceback": None,
+                    }
+                ],
+                "console": [],
+            },
+        ],
+    }
+    errors = nbi.parse_snapshot(_write(tmp_path, "cascade.py.json", snap))
+    assert [e["cell"] for e in errors] == ["MJUe"]
+
+
 def test_parse_snapshot_missing_file_reports_export_error(tmp_path):
     errors = nbi.parse_snapshot(tmp_path / "never-written.py.json")
     assert len(errors) == 1