feat: add eBPF flow collector with helper binary contract and agent integration

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

backend/app/api/v1/router.py

@@ -663,6 +663,11 @@ def compact_agent_payload(payload: AgentHeartbeat) -> dict:
         value["flow_count"] = len(flows)
         value["flows"] = flows[:50]
         value["flows_truncated"] = len(flows) > 50
+    ebpf_flows = value.get("ebpf_flows")
+    if isinstance(ebpf_flows, list):
+        value["ebpf_flow_count"] = len(ebpf_flows)
+        value["ebpf_flows"] = ebpf_flows[:50]
+        value["ebpf_flows_truncated"] = len(ebpf_flows) > 50
     return value
 
 
@@ -1417,6 +1422,10 @@ cat > "$CONFIG_DIR/config.json" <<'JSON'
   "node_name": "{node.name}",
   "interval_seconds": 30,
   "flow_limit": 1500,
+  "ebpf_collector": true,
+  "ebpf_binary": "/opt/nexafabric-agent/nexafabric-ebpf",
+  "ebpf_window_seconds": 10,
+  "ebpf_timeout_seconds": 15,
   "packet_flow_collector": true,
   "packet_flow_window_seconds": 10,
   "firewall_log_collector": true,