Fix deadlock in beginJob when compiled debug

[Copilot]

Debug-compiled EventDisplay deadlocks in beginJob() at cv_.wait(lock). GDB shows only 2 threads with the app thread apparently never reaching its event loop.

Root cause: ABBA lock ordering between m_ (held by beginJob) and gSystemMutex (acquired inside XThreadTimer constructor via R__LOCKGUARD2). The app thread holds gSystemMutex in gSystem->ProcessEvents() while timer callbacks attempt to acquire m_. Debug timing makes this race deterministic. Additionally, bare cv_.wait() without a predicate loses notifications if the signal arrives before the wait begins.

Changes:

• Restructured beginJob() to only hold m_ during cv_.wait(), not during XThreadTimer construction — eliminates lock ordering cycle
• Added appStarted_ and eveSetupDone_ predicate flags for the two synchronization points
• Switched to cv_.wait(lock, predicate) — handles both missed notifications and spurious wakeups
• signalAppStart() and setup_eve() set their respective flags under lock before notifying

// Before: holds m_ across timer creation → lock ordering deadlock
std::unique_lock lock{m_};
appThread_ = std::thread{[this] { run_application(); }};
XThreadTimer sut([this]{ signalAppStart(); });  // acquires gSystemMutex
cv_.wait(lock);

// After: m_ only held for the wait
appThread_ = std::thread{[this] { run_application(); }};
XThreadTimer sut([this]{ signalAppStart(); });
{
    std::unique_lock lock{m_};
    cv_.wait(lock, [this]{ return appStarted_; });
}

[brownd1978]

This PR does not change the behavior: Mu2eEventDisplay complied debug still hangs in beginJob at line 273.