feat(v2): add guarded execution state transitions

backend/src/backup_tool/execution.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+from collections.abc import Mapping
+
+ACTIVE_STATES = frozenset({"queued", "preparing", "running", "verifying", "cancelling"})
+TERMINAL_STATES = frozenset({"committed", "cancelled", "failed"})
+
+_ALLOWED: Mapping[str, frozenset[str]] = {
+    "queued": frozenset({"preparing", "cancelled", "failed"}),
+    "preparing": frozenset({"running", "cancelling", "failed"}),
+    "running": frozenset({"verifying", "cancelling", "failed"}),
+    "verifying": frozenset({"committed", "failed"}),
+    "cancelling": frozenset({"cancelled", "failed"}),
+    "committed": frozenset(),
+    "cancelled": frozenset(),
+    "failed": frozenset(),
+}
+
+
+class TransitionError(ValueError):
+    code = "invalid_transition"
+
+
+def transition(current: str, target: str) -> str:
+    """Validate and return one durable execution-state transition."""
+    if target not in _ALLOWED.get(current, frozenset()):
+        raise TransitionError(f"Cannot transition execution from {current!r} to {target!r}.")
+    return target