Extend cxplat safe integer wrappers

[Alan-Jowett]

Summary

extend the cxplat safe integer wrapper surface from size_t to the fixed-width integer families

• back the winuser and winkernel implementations directly with IntSafe and NtIntSafe helpers, enabling signed helper support in those translation units
• add regression coverage in both cxplat_test and usersim_tests, and update the checked-in project files for the .cpp wrapper sources

Testing

• cmake --build Q:\usersim\build --config Debug --target cxplat_test cxplat_winkernel usersim_tests -- /m
• Q:\usersim\build\bin\Debug\cxplat_test.exe [size]
• Q:\usersim\build\bin\Debug\usersim_tests.exe [cxplat][size]