M1: compat layer + pf.ko skeleton builds against Linux 7.3.4

- pfk.h: BSD types/ioctl encoding (OpenBSD bit layout), locks (NET_LOCK=rwsem),
  pools, timeouts, mbuf shim over sk_buff, ifnet registry, toeplitz/siphash
- pfk_mod/pfk_hooks: misc /dev/pf (0600), IPv4 netfilter hook skeleton,
  DIOCGETSTATUS answered (M1 skeleton; engine ioctls land with pf_ioctl.c)
- vendorize/stage-port scripts: deterministic upstream staging with seds
  (struct renames, IFF_*->PFIFF_*, sa_len drops) logged to docs/porting-notes
- fixes: modules_prepare+Module.symvers for out-of-tree builds

lab/cloud-init/network-config.pf0.tmpl

@@ -0,0 +1,16 @@
+# cloud-init network config v2 — pftest-pf0. mgmt=dhcp on default net,
+# data ifaces static (lab/ips.env). Rendered by scripts/vm-create.sh.
+version: 2
+ethernets:
+  mgmt:
+    match: {macaddress: "@PF0_MAC_MGMT@"}
+    dhcp4: true
+  lan:
+    match: {macaddress: "@PF0_MAC_LAN@"}
+    addresses: [@PF0_LAN@]
+  wan:
+    match: {macaddress: "@PF0_MAC_WAN@"}
+    addresses: [@PF0_WAN@]
+  wan2:
+    match: {macaddress: "@PF0_MAC_WAN2@"}
+    addresses: [@PF0_WAN2@]