M1 progress: engine port compiles except pf.c; AF_DIVERT socket family; ifq framework

- compat layer extended through the full BSD API surface pf_ioctl/pf_lb/
  pf_if/pf_norm/pf_table/pf_ruleset/radix/md5/sha2 need (all compile):
  cpumem/counters model, ifaddr/ifg groups, cred view, endian+in6 names,
  NAT46/64, SipHash24, mbuf_list, frag6 helpers, ip6 forward/checksum,
  tcp options/FSM names, taskqueue & sysctl shims (documented in
  docs/porting-notes.md)
- pfsrc/divert_sock.c: real BSD divert(4) as runtime-registered PF_DIVERT=32
  socket family inside the module (sock_register; NPROTO=AF_MAX=45 -> zero
  core patches); divert_packet/divert6_packet wired for pf.c
- compat/pfk_ifq.c: OpenBSD 8.0 ifq framework with priq back end; hfsc and
  fq_codel aliases pending M5 (recorded in feature matrix as pending)
- design + plan updated for the divert decision (user-requested)
- stage-port.sh: single deterministic rename program for headers and .c

module/include/compat/sys/mutex.h

@@ -0,0 +1,34 @@
+/* PFLINUX compat: <sys/mutex.h> — OpenBSD struct mutex over a Linux
+ * counting semaphore. BSD entry points are MACROS calling pfk_* helpers,
+ * after #undef of any same-named kernel macros (the kernel's own mutex_*
+ * functions were expanded before pfk.h and are unaffected). */
+#ifndef _PFK_SYS_MUTEX_H
+#define _PFK_SYS_MUTEX_H
+
+#define IPL_SOFTNET	6
+
+struct pfk_mutex {
+	struct semaphore	s;
+	const char		*name;
+};
+
+void pfk_mutex_setup(struct pfk_mutex *m, const char *name);
+
+#define PFK_MUTEX_INIT(obj) \
+	{ .s = __SEMAPHORE_INITIALIZER(obj, 1), .name = #obj }
+
+#undef mutex_init
+#undef mutex_destroy
+#undef mutex_is_locked
+#define mutex_init(m, n, f)		pfk_mutex_setup((m), (n))
+#define mutex_enter(m)			down(&(m)->s)
+#define mutex_leave(m)			up(&(m)->s)
+#define mutex_try_enter(m, t)		(down_trylock(&(m)->s) == 0)
+#define mutex_destroy(m)		do { } while (0)
+#define mutex_is_locked(m)		(atomic_long_read(&(m)->s.count) < 1)
+
+/* MUTEX_INITIALIZER(ipl) used in static definitions:
+ *   struct mutex x = MUTEX_INITIALIZER(IPL_SOFTNET); */
+#define MUTEX_INITIALIZER(ipl)		PFK_MUTEX_INIT(pfk_mtx_static)
+
+#endif