Support governed train-only corpus shards

backend/tests/test_sprint130_operator_yolo_tile_dataset.py

@@ -142,6 +142,15 @@ def test_manifest_sample_selection_keeps_external_holdouts_out_of_targeted_datas
         module.select_manifest_samples(samples, {"geel", "missing"})
 
 
+def test_train_only_shard_requires_explicit_opt_in() -> None:
+    module = load_tile_exporter()
+    samples = [{"sample_slug": "new-train-aoi", "recommended_split": "train"}]
+
+    with pytest.raises(SystemExit, match="must include at least one"):
+        module.validate_validation_split(samples, set())
+    assert module.validate_validation_split(samples, set(), allow_empty=True) == set()
+
+
 def test_validation_coverage_reports_holdouts_without_retained_tiles() -> None:
     module = load_tile_exporter()
     coverage = module.validation_sample_coverage(