docs: classify test planning documents

tests/tools/build_oversized_test_split_plan.py

@@ -443,6 +443,10 @@ def write_report(metrics: list[FileMetric], node_count_total: int) -> None:
         "",
         "> [!NOTE]",
         "> This is generated planning material based on the current test tree.",
+        ">",
+        "> It is non-canonical and disposable. Durable testing policy belongs in",
+        "> `tests/TESTING_STANDARD.md`; active split selection and prioritization",
+        "> belong in approved issues or the project tracker.",
         "> It does not define runtime behaviour or approve any test-file split.",
         "",
         "## Purpose",