add test module

sentence_api/tests/test_store.py

@@ -0,0 +1,7 @@
+from sentence_api.store import BoundaryStore
+
+
+def test_missing_legacy_index_starts_empty(tmp_path):
+    store = BoundaryStore(tmp_path / "not-created-yet.json")
+
+    assert store.count() == 0