Parallel scheduler namespace problems

[vasama]

The STDEXEC namespace is dependent on the definition of STDEXEC_NAMESPACE, which may be std::execution.

In __config.hpp there exists an unconditional definition of namespace std::execution::parallel_scheduler_replacement. It's unclear to me if this is intended.

In sudoku.cpp STDEXEC_NAMESPACE is locally defined as std::execution. However, this breaks when using STDEXEC_BUILD_PARALLEL_SCHEDULER , because parallel_scheduler_replacement is now built in a separate TU, which is unaware of the local namespace override. This nested namespace ends up in namespace STDEXEC in on TU and in namespace std::execution in another.

I also noticed that STDEXEC_BUILD_PARALLEL_SCHEDULER is not tested in CI, which is probably why this problem has gone unnoticed thus far.

I think the simplest solution here would be to:

1. Use STDEXEC instead of std and nested namespace in sudoku.cpp.
2. Define STDEXEC_BUILD_PARALLEL_SCHEDULER in the CI builds.
3. Either remove or document the unconditional std::execution namespace in __config.hpp.

[vasama]

Oh, now I also noticed that many of the tests and examples fail to build when STDEXEC_NAMESPACE is std::execution.

[vasama]

parallel_scheduler.lib/libparallel_scheduler.a contains query_parallel_scheduler_backend in namespace stdexec, while sudoku.obj/sudoku.cpp.o contains it it in namespace std::execution. Using MSVC, this fails to link due to the missing symbol. Using GCC and Clang, which apply a weak attribute, linking succeeds but example.sudoku crashes at runtime due to the missing symbol, whose absence was accepted due to the weak attribute.