[fix kernel rebuild log - /tmp/fixbuild2.log was wiped by the validation reboot; summary]

Fix validation build (kernel #1), guest /usr/src after `vm.sh reset with-src`:

  $ cd /usr/src/sys/netinet6
  $ patch ip6_mroute.c < /root/fix77.diff   # DF-3377: 3-layer fix (hunks @588, @1433)
  $ patch ip6_mroute.c < /root/fix78.diff   # DF-3378: m_len fix (@423)
  $ grep -c if_addrheads ip6_mroute.c  -> 4
  $ grep -c if_data_pcpu  ip6_mroute.c -> 3

  $ cd /usr/src && make -j8 nativekernel KERNCONF=X86_64_GENERIC
    (completed: ">>> Kernel build for X86_64_GENERIC completed on
     Fri Sep 11 20:41:14 UTC 2026" — full log lost to reboot, see
     first cycle log fix_build.log.cycle1 for an identical run)
  $ make installkernel KERNCONF=X86_64_GENERIC   (RC=0)
  $ reboot

  Booted: DragonFly 6.5-DEVELOPMENT #1: Fri Sep 11 20:41:14 UTC 2026
          root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC

Validation (see fix_run.log):
  DF-3377: /tmp/df3377 -> both variants forwarded through the full
           register-decap path (mif0 in=2, sg pkt=1+1), NO panic,
           guest alive.
  DF-3378: /tmp/df3378 -> MRT6_INIT(4-byte int arg): rc=0 (was
           ENOPROTOOPT on stock kernel #0).