feat(v2): complete v2 reimplementation

tests/integration/test_empty_source.py

@@ -0,0 +1,13 @@
+from __future__ import annotations
+
+import importlib
+
+import pytest
+
+snapshot = importlib.import_module("backup_tool.snapshot")
+
+
+def test_empty_source_requires_explicit_opt_in() -> None:
+    with pytest.raises(snapshot.SnapshotError, match="source_empty"):
+        snapshot.require_nonempty([], False)
+    snapshot.require_nonempty([], True)