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/package.json

@@ -35,6 +35,7 @@
   },
   "private": true,
   "dependencies": {
+    "check-disk-space": "^3.4.0",
     "cron-parser": "^5.5.0",
     "extract-zip": "^2.0.1",
     "yaml": "^2.8.3"