Add nexafabric-ebpf.go implementing flow collection via Linux raw packet sockets (AF_PACKET) instead of tc/eBPF to enable immediate Proxmox deployment without kernel dependencies, implement packet parsing with VLAN/IP/TCP/UDP/ICMP support and flow aggregation by 5-tuple with vmid/nic/interface/direction metadata extraction from tap/fwbr interface names, add /agents/download/nexafabric-ebpf.go endpoint
