P3016R6: Resolve inconsistencies in begin/end for `valarray` and braced initializer lists

**Link:** https://wg21.link/P3016R6

Feature-test macros:
```c++
#define __cpp_lib_initializer_list                  202511L // freestanding, also in <initializer_list>
#define __cpp_lib_valarray                          202511L // also in <valarray>
```

This paper should be treated as a Defect Report and applied to old modes. It resolves LWG3537, LWG3624, LWG3625, and LWG4131.