[string variant, kernel #0, unprivileged uid 1001: NO console output at all]
mode=string path=/boot pid=...

- ssh connection times out; vm.sh status => down; QEMU process alive at
  102% CPU (ps -p <pid> -o pcpu => 102 qemu-system-x86_64); serial console
  shows a fully booted system frozen at "login:" with nothing after.
- Mechanism: pa_rwlock aliases ps_mutable (heap pointer; low 32 bits are
  never 0) -> mtx_lock fast cmpset always fails -> __mtx_lock_ex retry
  loop spins forever in kernel mode with the mount's ac_spin held.  No
  trap, no panic, no scheduling: full-system livelock from ONE
  unprivileged syscall.