P1494R5 Partial Program Correctness

[StephanTLavavej]

WG21-P1494R5 Partial Program Correctness
WG21-P3641R0 observable_checkpoint()

WG21-P1494R5 is a Core paper with Library impact. WG21-P3641R0 was voted in as a purely Library paper.

Feature-test macro (expected):

#define __cpp_lib_observable_checkpoint 202506L

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.