feat(FN-1227): add mesh state and system metrics support to CentralCore

- Add SystemMetrics, PeerNode, NodeMeshState, and MeshDiscovery types and export new mesh/metrics APIs from @fusion/core
- Introduce collectSystemMetrics with CPU, memory, storage, and uptime collection (including check-disk-space integration) plus dedicated unit tests
- Bump central DB schema to v3 with nodes.systemMetrics/knownPeers and peerNodes table, including migration coverage for v2->v3 upgrades
- Extend CentralCore with node metrics updates, peer register/unregister/list operations, mesh snapshot reporting, and event emissions backed by expanded test coverage

packages/core/src/system-metrics.test.ts

@@ -0,0 +1,71 @@
+import { describe, it, expect, beforeEach, vi } from "vitest";
+import { collectSystemMetrics } from "./system-metrics.js";
+
+const { checkDiskSpaceMock } = vi.hoisted(() => ({
+  checkDiskSpaceMock: vi.fn(),
+}));
+
+vi.mock("check-disk-space", () => ({
+  default: checkDiskSpaceMock,
+}));
+
+describe("collectSystemMetrics", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    checkDiskSpaceMock.mockResolvedValue({
+      diskPath: "/",
+      free: 250_000,
+      size: 1_000_000,
+    });
+  });
+
+  it("returns a valid SystemMetrics object", async () => {
+    const metrics = await collectSystemMetrics();
+
+    expect(metrics).toEqual(
+      expect.objectContaining({
+        cpuUsage: expect.any(Number),
+        memoryUsed: expect.any(Number),
+        memoryTotal: expect.any(Number),
+        storageUsed: expect.any(Number),
+        storageTotal: expect.any(Number),
+        uptime: expect.any(Number),
+        reportedAt: expect.any(String),
+      }),
+    );
+  });
+
+  it("returns cpuUsage between 0 and 100", async () => {
+    const metrics = await collectSystemMetrics();
+    expect(metrics.cpuUsage).toBeGreaterThanOrEqual(0);
+    expect(metrics.cpuUsage).toBeLessThanOrEqual(100);
+  });
+
+  it("returns memoryUsed less than or equal to memoryTotal", async () => {
+    const metrics = await collectSystemMetrics();
+    expect(metrics.memoryUsed).toBeLessThanOrEqual(metrics.memoryTotal);
+  });
+
+  it("returns storageUsed less than or equal to storageTotal", async () => {
+    const metrics = await collectSystemMetrics();
+    expect(metrics.storageUsed).toBeLessThanOrEqual(metrics.storageTotal);
+  });
+
+  it("returns uptime greater than 0", async () => {
+    const metrics = await collectSystemMetrics();
+    expect(metrics.uptime).toBeGreaterThan(0);
+  });
+
+  it("returns a valid ISO timestamp in reportedAt", async () => {
+    const metrics = await collectSystemMetrics();
+    expect(new Date(metrics.reportedAt).toISOString()).toBe(metrics.reportedAt);
+  });
+
+  it("passes dbPath through to check-disk-space", async () => {
+    const customPath = "/tmp/kb-metrics-db";
+
+    await collectSystemMetrics(customPath);
+
+    expect(checkDiskSpaceMock).toHaveBeenCalledWith(customPath);
+  });
+});