Fixes#106
The Nix settings system uses global mutable state without internal
synchronization. When multiple threads call settings::set concurrently
(as happens in parallel test execution), this causes a segfault in the
C++ std::set implementation.
Changes:
- Add mutex to serialize access through the Rust API
- Add documentation explaining thread safety limitations
- Add Once guard in nix-flake tests to minimize concurrent access
The mutex provides protection between Rust callers, though it cannot
completely prevent C++ Nix code from modifying settings concurrently.
(cherry picked from commit 203917657b60c4e1dcbaf442bec64c37c634abc4)
