Stop and start a flow without rebuilding anything

A stopped flow's nodes are built like any other flow's — being stopped
means having no subscriptions, schedules or webhooks, not being absent —
so a toggle only ever needed the lifecycle call and the gate that goes
with it. It was doing a whole-pipeline rebuild instead, which on a
populated installation is every node in every flow reconnecting.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01StpRc2C6au1WJ1EUU7fsfu

backend/tests/flow/test_runtime_control.py

@@ -297,3 +297,38 @@ def test_a_deleted_flow_leaves_its_consumers_reporting_a_missing_input(
         assert engine.pipeline.get_node_by_id("a.meter") is None
 
     asyncio.run(asyncio.wait_for(scenario(), timeout=10))
+
+
+def test_stopping_one_flow_does_not_touch_the_others(
+    tmp_path: Path, lifecycle: list[str]
+):
+    """A toggle is a lifecycle call, not a rebuild.
+
+    A stopped flow's nodes are still built — being stopped means having no
+    subscriptions, not being absent — so nothing is compiled and no other
+    flow's node is asked to reconnect. That is where the seconds went.
+    """
+    store = FlowStore(tmp_path / "flows")
+    for name in ("a", "b"):
+        store.write_flow(FlowDef(name=name, nodes=[NodeDef(id="io", type="lifecycle")]))
+    engine = FlowController(store)
+
+    async def scenario() -> None:
+        await engine.reload()
+        before = {name: engine.loaded[name].node for name in ("a.io", "b.io")}
+        lifecycle.clear()
+
+        await engine.set_enabled("a", False)
+
+        assert lifecycle == ["stop a.io"]
+        # Still built, still the same objects: only the lifecycle changed.
+        assert {name: engine.loaded[name].node for name in ("a.io", "b.io")} == before
+        assert engine.disabled == {"a"}
+
+        lifecycle.clear()
+        await engine.set_enabled("a", True)
+
+        assert lifecycle == ["start a.io"]
+        assert {name: engine.loaded[name].node for name in ("a.io", "b.io")} == before
+
+    asyncio.run(asyncio.wait_for(scenario(), timeout=10))