Parallel scheduler namespace problems

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.

Start by comparing the namespace handling in __config.hpp and sudoku.cpp, then inspect CI configurations for STDEXEC_BUILD_PARALLEL_SCHEDULER. Reproduce the separate translation-unit build and verify that parallel_scheduler_replacement uses a consistent namespace. Done means the scheduler build succeeds and CI covers the option, with the unconditional namespace documented or removed.