Add BufWriter::with_capacity(64KB) to generate_test_file,
generate_growable_file, and append_lines in data_gen.rs.
Previously each writeln! triggered an individual write syscall,
making 5GB/74M-line benchmark data generation extremely slow.
BufWriter batches writes into 64KB chunks, reducing syscalls
by ~1000x.
Explicit flush()? + drop before subsequent reads ensures data
visibility and propagates flush errors (BufWriter::drop swallows
them).
Closes#35
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the handler-active-but-state-unpublished race window)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
