=== PATCHED-SHAPE RUN: fix.diff protocol (try-enter + 1-tick retry) ===
[guest] kldload /root/fxpkmod_fixed/fxp_callout_replica_fixed.ko
[guest] sysctl kern.fxrf.arm=1; sleep 2
[guest] sysctl kern.fxrf.stop_blk=1   (same fxp_stop shape, 3s serializer hold)
[guest] kldunload fxp_callout_replica_fixed

--- serial console decisive lines (current boot, post with-src reset):
login: fxp_callout_replica_fixed: loaded (hz=100)
fxrf: armed (FIXED fxp_tick shape)
fxrf: holding serializer, callout_stop() [fxp_stop shape]...
fxrf: callout_stop RETURNED cleanly, pending=0

kldunload: FIXED_SHAPE_OK (no wedge, module unloaded)