feat(v2): add guarded execution state transitions

tests/unit/test_execution_lifecycle.py

@@ -0,0 +1,23 @@
+from __future__ import annotations
+
+from backup_tool.execution import ACTIVE_STATES, TERMINAL_STATES, TransitionError, transition
+
+
+def test_transitions_are_monotonic_and_terminal_states_cannot_change() -> None:
+    assert transition("queued", "preparing") == "preparing"
+    assert transition("preparing", "running") == "running"
+    assert transition("running", "verifying") == "verifying"
+    assert transition("verifying", "committed") == "committed"
+    assert "committed" in TERMINAL_STATES
+    assert "queued" in ACTIVE_STATES
+    try:
+        transition("committed", "cancelled")
+    except TransitionError as error:
+        assert error.code == "invalid_transition"
+    else:
+        raise AssertionError("terminal execution changed state")
+
+
+def test_cancellation_is_cooperative() -> None:
+    assert transition("running", "cancelling") == "cancelling"
+    assert transition("cancelling", "cancelled") == "cancelled"