feat(FN-1224): add peer gossip protocol for mesh network synchronization
- Add peer exchange types (MeshSyncState, PeerExchangeMessage, PeerGossipConfig) to core types
- Add peer merge and sync methods to CentralCore (mergeProject, syncWithPeer, getMeshState)
- Add mesh sync API endpoints (GET /api/mesh/state, POST /api/mesh/sync)
- Implement PeerExchangeService background gossip engine for periodic peer synchronization
- Add comprehensive tests for CentralCore mesh methods and PeerExchangeService
- Update memory docs with peer gossip protocol documentation
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