Commit 48dcaace7 introduced PrimedSharedMutex and converted every host std::shared_mutex that existed at the time, because winpthreads initializes a static rwlock on first use and a concurrent first lock returns EINVAL, which std::shared_mutex ignores (docs/dev/TechnicalDebt.md:10).
Two host mutexes were added afterward and missed the conversion:
- Recompiler.cpp registrationModule() static mutex (added by baf3437465)
- MultisampleTarget.cpp guardedMutex() static mutex (added by d109f3966f)
Apply the same priming. Recompiler.cpp reuses the PrimedSharedMutex already defined in that translation unit; MultisampleTarget.cpp gets the identical local helper to match the existing inline precedent.
Tested: Windows 11 x64, WinLibs GCC 15.2.0 (matches CI). Compiled Recompiler.cpp.obj and all three MultisampleTarget.cpp.obj targets with the project build flags; check_conventions.py passes.