P3982R2: Split `strided_slice` into `extent_slice` and `range_slice` for C++26

**Link:** https://wg21.link/P3982R2

Feature-test macro (updated):
```c++
#define __cpp_lib_submdspan 202603L // freestanding, also in
```

Start by reading the linked P3982R2 proposal to understand the requested C++26 API split and the updated __cpp_lib_submdspan value. Then locate the existing submdspan implementation and tests in the repository; done means the proposal's extent_slice and range_slice interfaces are supported and the feature-test macro is updated to 202603L.

Written by the indexing model from the issue text.