dad5f5a635
Replace `static mut OLD_SIGBUS_HANDLER` with AtomicU8 + AtomicPtr to remove data race UB when concurrent benchmarks call open() from multiple threads. Key changes: - Use `Once::call_once` to guarantee single handler installation - Publish old handler to atomics BEFORE installing new handler (closes the handler-active-but-state-unpublished race window) - Read atomics with Acquire in signal handler (async-signal-safe) - Align si_addr to page boundary before mmap(MAP_FIXED) - Add concurrent test: 8 threads open all 5 variants simultaneously