Step a paused flow, and deliver what a rate limit held back

Three things a pause and an interval were quietly losing:

- A rebuild builds a fresh pipeline, so nothing is paused any more and no
  resume ever comes for what the old one parked. Release it on rebuild.
- POST /flows/{name}/step takes the oldest parked item and runs that one wave
  while the flow stays paused, so a held-back cascade can be walked through.
  Nothing parked answers plainly rather than failing.
- A per-port interval was leading-edge only: a producer going quiet inside the
  window left the consumer on the value before it. The held value is kept and
  a flush item scheduled on the queue's existing timer, so the window ends with
  a delivery. One timer in flight per node, and none without a queue to run it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H7LwYgJfpkbLCTeiAf8U4A

backend/tests/flow/test_runtime_control.py

@@ -1,10 +1,17 @@
 """Stopping a flow takes it off the engine; pausing holds its nodes."""
 
 from pathlib import Path
+from typing import Any
 
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+from app.api.deps import get_current_user
+from app.api.routes.flows import router
 from app.flow.messages import DType, MessageSpec
 from app.flow.nodes import Node
 from app.flow.pipeline import Pipeline
+from app.flow.schemas import FlowDef
 from app.flow.store import FlowStore
 
 
@@ -101,3 +108,60 @@ def test_stopped_survives_a_restart(tmp_path: Path):
 
     # A second store over the same directory is what a restart looks like.
     assert FlowStore(tmp_path / "flows").read_enabled("heating") is False
+
+
+class _StubController:
+    """Only what the step route touches: the store, and the step itself."""
+
+    def __init__(self, store: FlowStore, stepped: str | None) -> None:
+        self.store = store
+        self.stepped = stepped
+        self.calls: list[str] = []
+
+    def step_flow(self, flow: str) -> str | None:
+        self.calls.append(flow)
+        return self.stepped
+
+
+def _stub(tmp_path: Path, stepped: str | None) -> _StubController:
+    """A controller over a store holding one flow, called 'heating'."""
+    store = FlowStore(tmp_path / "flows")
+    store.write_flow(FlowDef(name="heating"))
+    return _StubController(store, stepped)
+
+
+def _client(controller: Any) -> TestClient:
+    app = FastAPI()
+    app.include_router(router, prefix="/api/v1")
+    app.state.flow_controller = controller
+    app.dependency_overrides[get_current_user] = lambda: None
+    return TestClient(app)
+
+
+def test_the_step_endpoint_reports_what_it_ran(tmp_path: Path):
+    controller = _stub(tmp_path, "heating.sensor")
+
+    response = _client(controller).post("/api/v1/flows/heating/step")
+
+    assert response.status_code == 200
+    assert response.json() == {"message": "Stepped 'heating.sensor'"}
+    assert controller.calls == ["heating"]
+
+
+def test_stepping_a_flow_with_nothing_parked_answers_plainly(tmp_path: Path):
+    """A button pressed once too often is not an error."""
+    controller = _stub(tmp_path, None)
+
+    response = _client(controller).post("/api/v1/flows/heating/step")
+
+    assert response.status_code == 200
+    assert response.json() == {"message": "Nothing held back in flow 'heating'"}
+
+
+def test_stepping_an_unknown_flow_is_a_404(tmp_path: Path):
+    controller = _stub(tmp_path, None)
+
+    response = _client(controller).post("/api/v1/flows/nosuchflow/step")
+
+    assert response.status_code == 404
+    assert controller.calls == []