SIGSEGV on simulation exit

Happening in 2024.2.1, seen in CI. Really hard to reproduce locally, but I caught this:

(gdb) bt
#0  ___pthread_mutex_lock (mutex=0x7f5cac038) at pthread_mutex_lock.c:80
#1  0x00007f5cb569a7e0 in __gthread_mutex_lock (__mutex=0x7f5cac038)
    at /usr/include/x86_64-linux-gnu/c++/11/bits/gthr-default.h:749
#2  std::mutex::lock (this=0x7f5cac038) at /usr/include/c++/11/bits/std_mutex.h:100
#3  std::scoped_lock<std::mutex>::scoped_lock (__m=..., this=<synthetic pointer>)
    at /usr/include/c++/11/mutex:655
#4  HAL_StopNotifier (notifierHandle=<optimized out>, status=<optimized out>)
    at /work/hal/src/main/native/sim/Notifier.cpp:200
#5  0x00007f5cb122f541 in (anonymous namespace)::HeartbeatDaemon::Main() ()
   from /home/virtuald/src/frc/robotpy-rev/rev/_rev.cpython-38-x86_64-linux-gnu.so
#6  0x00007f5cb5939c4b in operator() (__closure=0x5587df615988)
    at /work/wpiutil/src/main/native/cpp/SafeThread.cpp:79
#7  std::__invoke_impl<void, wpi::detail::SafeThreadOwnerBase::Start(std::shared_ptr<wpi::SafeThreadBase>)::<lambda()> > (__f=...) at /usr/include/c++/11/bits/invoke.h:61
#8  std::__invoke<wpi::detail::SafeThreadOwnerBase::Start(std::shared_ptr<wpi::SafeThreadBase>)::<lambda()> > (__fn=...) at /usr/include/c++/11/bits/invoke.h:96
#9  std::thread::_Invoker<std::tuple<wpi::detail::SafeThreadOwnerBase::Start(std::shared_ptr<wpi::SafeThreadBase>)::<lambda()> > >::_M_invoke<0> (this=0x5587df615988) at /usr/include/c++/11/bits/std_thread.h:259
#10 std::thread::_Invoker<std::tuple<wpi::detail::SafeThreadOwnerBase::Start(std::shared_ptr<wpi::SafeThreadBase>)::<lambda()> > >::operator() (this=0x5587df615988) at /usr/include/c++/11/bits/std_thread.h:266
#11 std::thread::_State_impl<std::thread::_Invoker<std::tuple<wpi::detail::SafeThreadOwnerBase::Start(std::shared_ptr<wpi::SafeThreadBase>)::<lambda()> > > >::_M_run(void) (this=0x5587df615980)
    at /usr/include/c++/11/bits/std_thread.h:211
#12 0x00007f5cb54e31e3 in std::execute_native_thread_routine (__p=0x5587df615980)
    at ../../../../../libstdc++-v3/src/c++11/thread.cc:104
#13 0x00007f5cc36ac897 in start_thread (arg=<optimized out>) at pthread_create.c:444
#14 0x00007f5cc373380c in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78

The trace points to HAL_StopNotifier in /work/hal/src/main/native/sim/Notifier.cpp:200, called from HeartbeatDaemon::Main and SafeThread.cpp:79. Read the notifier and thread shutdown paths first, then investigate the CI-only exit failure; done means simulation exits without a SIGSEGV.