DF-4794 fix compile-validation + logic-demo session on the audit guest
(2026-09-23), full untrimmed transcript (run via dfbsd-qemu/vm.sh run_root).

[1] Verify guest source == audited tree
$ sha256 /usr/src/sys/dev/netif/ig_hal/e1000_vf.c
SHA256 (/usr/src/sys/dev/netif/ig_hal/e1000_vf.c) = c0cdce6b90e860665142e690ee34ffbf0f70332a2c6cb45a3719a4c217d19dd2

[2] Apply exact hunks of fix.diff to a copy in /tmp/vfchk
$ cp e1000_vf.c e1000_vf_base.c && patch -p1 < fix.diff
Hmm...  Looks like a unified diff to me...
Patching file e1000_vf.c using Plan A...
Hunk #1 succeeded at 400.
Hunk #2 succeeded at 419.
done
PATCH_RC=0

[3] Compile-validate baseline and patched
$ cc -fsyntax-only -D_KERNEL -I/usr/obj/usr/src/sys/X86_64_GENERIC \
     -I/usr/src/sys -I/usr/src/sys/dev/netif/ig_hal e1000_vf_base.c
BASE_CC_RC=0
(only pre-existing warning: /usr/src/sys/sys/systm.h:216: conflicting types
 for built-in function 'log' [-Wbuiltin-declaration-mismatch])

$ cc -fsyntax-only -D_KERNEL -I/usr/obj/usr/src/sys/X86_64_GENERIC \
     -I/usr/src/sys -I/usr/src/sys/dev/netif/ig_hal e1000_vf.c
FIXED_CC_RC=0
(same single pre-existing systm.h warning)

$ sha256 e1000_vf_base.c e1000_vf.c
SHA256 (e1000_vf_base.c) = c0cdce6b90e860665142e690ee34ffbf0f70332a2c6cb45a3719a4c217d19dd2
SHA256 (e1000_vf.c)      = c3997db5a00621f057a5d2bb96991047f765bcf18a104da1dbd8964c6fe36d36

[4] Logic demonstrator (see run.log for its output)
$ cc -Wall -O2 -o mcmsg_demo mcmsg_demo.c
BUILD_RC=0

$ rm -rf /tmp/vfchk && echo CLEANED
CLEANED