P2630R4 `submdspan`

[StephanTLavavej]

WG21-P2630R4 submdspan
WG21-P2642R6 Padded mdspan Layouts
WG21-P3355R1 Fix submdspan
WG21-P3663R3 Future-Proof submdspan_mapping
LWG-3970 [mdspan.syn] Missing definition of full_extent_t and full_extent
LWG-4060 submdspan preconditions do not forbid creating invalid pointer
LWG-4272 For rank == 0, layout_stride is atypically convertible
LWG-4372 Weaken Mandates: for dynamic padding values in padded layouts

Feature-test macro (expected):

#define __cpp_lib_submdspan 202511L

Note: We're focused on implementing the remaining library-only features in C++23. Until that's done, we will NOT be accepting PRs for C++26 features.