feat(FN-3454): add typed mesh snapshot API consumed by nodes view

Implements a typed mesh snapshot API in `@fusion/core` (`CentralCore.getMeshSnapshot`, `MeshSnapshot` type) and wires it through the dashboard nodes view via a new `useMeshState` hook, substantially simplifying `MeshTopology.tsx` by replacing internal state management with the centralized snapshot.

Fusion-Task-Id: FN-3454

packages/dashboard/app/hooks/__tests__/useMeshState.test.ts

@@ -0,0 +1,73 @@
+import { act, renderHook } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { useMeshState } from "../useMeshState";
+import * as api from "../../api";
+
+vi.mock("../../api", () => ({
+  fetchMeshState: vi.fn(),
+}));
+
+const mockFetchMeshState = vi.mocked(api.fetchMeshState);
+
+async function flushPromises(): Promise<void> {
+  await Promise.resolve();
+  await Promise.resolve();
+}
+
+describe("useMeshState", () => {
+  beforeEach(() => {
+    vi.useFakeTimers({ shouldAdvanceTime: true });
+    mockFetchMeshState.mockReset();
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("loads mesh state on mount", async () => {
+    mockFetchMeshState.mockResolvedValueOnce({ collectedAt: "2026-01-01T00:00:00.000Z", sourceNodeId: "local", nodes: [] });
+    const { result } = renderHook(() => useMeshState());
+
+    await act(async () => {
+      await flushPromises();
+    });
+
+    expect(result.current.loading).toBe(false);
+    expect(result.current.meshState?.sourceNodeId).toBe("local");
+  });
+
+  it("refresh updates state", async () => {
+    mockFetchMeshState
+      .mockResolvedValueOnce({ collectedAt: "2026-01-01T00:00:00.000Z", sourceNodeId: "local", nodes: [] })
+      .mockResolvedValueOnce({ collectedAt: "2026-01-01T00:01:00.000Z", sourceNodeId: "local", nodes: [{ nodeId: "remote", nodeName: "Remote", nodeUrl: "http://remote", nodeType: "remote", status: "online", metrics: null, lastSeen: "2026-01-01T00:01:00.000Z", connectedAt: "2026-01-01T00:00:00.000Z", knownPeers: [] }] });
+
+    const { result } = renderHook(() => useMeshState());
+    await act(async () => {
+      await flushPromises();
+    });
+
+    await act(async () => {
+      await result.current.refresh();
+    });
+
+    expect(result.current.meshState?.nodes).toHaveLength(1);
+  });
+
+  it("retains stale mesh state on refresh error", async () => {
+    mockFetchMeshState
+      .mockResolvedValueOnce({ collectedAt: "2026-01-01T00:00:00.000Z", sourceNodeId: "local", nodes: [{ nodeId: "local", nodeName: "Local", nodeUrl: undefined, nodeType: "local", status: "online", metrics: null, lastSeen: "2026-01-01T00:00:00.000Z", connectedAt: "2026-01-01T00:00:00.000Z", knownPeers: [] }] })
+      .mockRejectedValueOnce(new Error("mesh unavailable"));
+
+    const { result } = renderHook(() => useMeshState());
+    await act(async () => {
+      await flushPromises();
+    });
+
+    await act(async () => {
+      await result.current.refresh();
+    });
+
+    expect(result.current.error).toBe("mesh unavailable");
+    expect(result.current.meshState?.nodes).toHaveLength(1);
+  });
+});