feat: add Go-based eBPF helper using AF_PACKET raw sockets with automatic build in agent installer

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

backend/app/agent_assets/EBPF_HELPER_CONTRACT.md

@@ -1,6 +1,9 @@
 # NexaFabric eBPF helper contract
 
 Agent 0.3.0 can call an optional helper binary at `/opt/nexafabric-agent/nexafabric-ebpf`.
+The repository includes a dependency-free Go helper source at `nexafabric-ebpf.go`.
+
+The first implementation uses Linux raw packet sockets on the selected VM/LXC interfaces and prints the same JSON contract that a tc/eBPF implementation should print. This keeps the helper installable on Proxmox immediately while preserving the agent integration point for a later kernel eBPF loader.
 
 The helper is invoked as:
 
@@ -28,7 +31,7 @@ It must print JSON to stdout:
     }
   ],
   "diagnostics": {
-    "attach_mode": "tc",
+    "attach_mode": "af_packet_raw_socket",
     "interfaces_attached": ["tap100i0"]
   }
 }