Coding - Replace Standard_Mutex with std::mutex and migrate to RAII locks (#766)

Coding - Replace Standard_Mutex with std::mutex and migrate to RAII locks (#766)

- Replace legacy Standard_Mutex usage across many modules with std::mutex.
- Include <mutex> where needed and remove <Standard_Mutex.hxx> includes.
- Replace Standard_Mutex::Sentry / explicit Lock/Unlock with std::lock_guard or std::unique_lock.
- Convert optional/heap mutex holders to std::unique_ptr<std::mutex> and adapt locking accordingly.
- Simplify several singleton initializations (remove manual double-checked locking where safe).
- Use thread_local for per-thread flags instead of ad-hoc mutex protection.
- Fix BVH_BuildQueue Fetch logic to preserve thread counters and wasBusy handling.
- Remove obsolete TopTools_MutexForShapeProvider sources and update FILES.cmake.

This modernizes mutex usage, reduces dependency on custom mutex types and improves clarity of locking patterns.