fix(bench): eliminate SIGBUS handler static mut UB with Once + raw atomics (closes #33)

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  the handler-active-but-state-unpublished race window)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

crates/bench/src/lib.rs

@@ -0,0 +1,24 @@
+pub mod data_gen;
+pub mod line_index;
+pub mod metrics;
+pub mod mmap_reader;
+pub mod pread_reader;
+pub mod report;
+pub mod runner;
+pub mod suites;
+pub mod types;
+
+use std::path::Path;
+
+/// A single reader backend (mmap or pread)
+pub trait FileReaderBackend {
+    fn name(&self) -> &str;
+    fn open(path: &Path) -> std::io::Result<Self>
+    where
+        Self: Sized;
+    fn file_size(&self) -> u64;
+    fn total_lines(&self) -> usize;
+    fn get_line(&self, idx: usize) -> Option<String>;
+    fn read_range(&self, offset: u64, len: usize) -> Option<Vec<u8>>;
+    fn close(self);
+}