testdata: seed sandbox overlap child

Stack Demo · Stack Demo · commit 033939113df5 · 2026-03-25T16:25:26.000-04:00
diff --git a/_test_data/sandbox/overlap/shared.md b/_test_data/sandbox/overlap/shared.md
@@ -6,6 +6,6 @@ Parent branch owns this section and introduces the file.
 
 ## Section B
 
-This section will be edited by the child branch.
+Child branch rewrites this section to create same-file stacked diffs without a guaranteed merge conflict.
 
-Marker: 74d5552
+Marker: b9b5c29
