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
Add EBPF_HELPER_CONTRACT.md documenting helper binary invocation with --json/--limit/--duration/--interfaces parameters and expected JSON output format with flows/diagnostics, implement collect_ebpf_flows to invoke helper binary with configurable timeout/window/interfaces and normalize flow fields (source_ip/destination_ip/protocol/ports/packets/bytes/state), add executable_exists and flow_int helpers for binary validation
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