Fix single-shot scheduler pointer race and guard stop_and_wait

[LimiNode]

Summary

• add a per-thread timer execution scope so stop_and_wait can assert when called reentrantly from inside the callback
• cache the TimerScheduler pointer before installing the single-shot lambda and reuse the cached value when cleaning up
• wrap due timer execution in an exception guard and ensure Timer::~Timer() stops cleanly before releasing its state

Testing

• cmake -S . -B build
• cmake --build build
• ctest --test-dir build

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https://chatgpt.com/codex/tasks/task_e_68d1ea4e844c832c94779ffe336ff33d