nx_mutex_race_test.nx

nx_mutex_race_test.nx -- prove the mutex provides REAL mutual exclusion under multi-thread contention. hw-derived (cpu_count * STRESS_FACTOR) worker threads each grab the same mutex N times and bump a PLAIN (non-atomic) counter inside the critical section. If the mutex doesn't actually exclude, the non-atomic increment races and final counter < N (lost updates). PASS criterion: final counter == n_threads * N exactly.