feat(ebpf): add nginx HTTP capture infrastructure via kretprobe recvfrom

Add supporting infrastructure for nginx HTTP capture using kretprobe
on __x64_sys_recvfrom to replace the blocked tracepoint sys_exit_recvfrom.

Changes:
- bpf/bpf_types.h: Add nginx_pid_map for filtering recvfrom by PID
- cmd/ja4ebpf/main.go: Add Uprobes configuration section
- Makefile: Add test targets for recvfrom validation
- internal/loader: Generate nginx HTTP event structures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/ja4ebpf/bpf/bpf_types.h

@@ -257,6 +257,14 @@ struct {
     __type(value, struct ssl_conn_info);
 } fd_conn_map SEC(".maps");
 
+/* Hash map : PID nginx → flag pour filtrage recvfrom (tracepoints) */
+struct {
+    __uint(type, BPF_MAP_TYPE_HASH);
+    __uint(max_entries, 16);
+    __type(key, __u32);
+    __type(value, __u8);
+} nginx_pid_map SEC(".maps");
+
 /* Hash map : pid_tgid → nginx_read_args (arguments read entry pour nginx) */
 struct {
     __uint(type, BPF_MAP_TYPE_HASH);