M2: live netfilter data path (pf_test per direction), pfattach bring-up, NET_LOCK reentrancy fix, static pfctl

- pfk_hooks: PRE→PF_IN, LOCAL_OUT/POST→PF_OUT v4+v6; replacement/buffer
  handling with NF_STOLEN + re-injection (documented divergence: re-entry)
- pfk_mod: pf_status_init/pfattach + pfi attach via pfk_if event callback
- pfk_kern: NET_LOCK holder reentrancy (fix deadlock when notifier fires
  under the write lock — observed as full-VM network hang)
- tools: static pfctl link for musl VMs

tools/include/sys/syslimits.h

@@ -0,0 +1,8 @@
+/* PFLINUX: <sys/syslimits.h> — glibc ships no such header; PATH_MAX etc.
+ * live in <limits.h>. */
+#ifndef _PFLINUX_SYS_SYSLIMITS_H
+#define _PFLINUX_SYS_SYSLIMITS_H
+
+#include <limits.h>
+
+#endif /* _PFLINUX_SYS_SYSLIMITS_H */