fix(oracle-r4): validate selectedStepId across live refreshes

Oracle r4 'Watch Out For' caught a real edge case: if a backend graph
changes shape while the scenarioId stays the same, the prior
selectedStepId could survive the merge and point at a step that no
longer exists. Two new vitest regressions in state/store.test.ts now
pin the contract:

- refreshLive() resets selectedStepId to the scenario's defaultStepId
  when the prior step no longer exists in the merged catalog
- refreshLive() preserves a still-valid selectedStepId

runLiveFetch() now derives stepStillThere from the merged scenario's
own steps (not the old store) and falls back to defaultStepId when
stale. Same single-set call as before; no extra renders.

Confidence: high
Scope-risk: narrow
Not-tested: real backend definition with step IDs that disappear
mid-session (covered by stub + assertion above)

src/state/store.test.ts

@@ -74,4 +74,28 @@ describe("store live-mode + refresh", () => {
     await useApp.getState().setMode("snapshot");
     expect(useApp.getState().toasts.length).toBe(before);
   });
+
+  it("refreshLive() resets selectedStepId to defaultStepId when the prior step no longer exists in the merged catalog", async () => {
+    stubFetch();
+    await useApp.getState().setMode("live");
+    const scenario = useApp.getState().scenarios[0];
+    if (!scenario) throw new Error("no scenario");
+    useApp.setState({ scenarioId: scenario.id, selectedStepId: "definitely-not-a-real-step-id" });
+    await useApp.getState().refreshLive();
+    const after = useApp.getState();
+    const sc = after.scenarios.find((s) => s.id === after.scenarioId);
+    expect(sc).toBeDefined();
+    expect(sc!.steps.some((st) => st.id === after.selectedStepId)).toBe(true);
+  });
+
+  it("refreshLive() preserves a still-valid selectedStepId", async () => {
+    stubFetch();
+    await useApp.getState().setMode("live");
+    const scenario = useApp.getState().scenarios[0];
+    if (!scenario) throw new Error("no scenario");
+    const validStep = scenario.steps[scenario.steps.length - 1];
+    useApp.setState({ scenarioId: scenario.id, selectedStepId: validStep.id });
+    await useApp.getState().refreshLive();
+    expect(useApp.getState().selectedStepId).toBe(validStep.id);
+  });
 });