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  the handler-active-but-state-unpublished race window)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/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "log-viewer-bench"
+version = "0.1.0"
+edition = "2024"
+
+[[bin]]
+name = "log-viewer-bench"
+path = "src/main.rs"
+
+[dependencies]
+memmap2 = "0.9"
+nix = { version = "0.30", features = ["signal", "resource", "mman", "fs"] }
+libc = "0.2"
+memchr = "2"
+serde_json = "1"
+clap = { version = "4", features = ["derive"] }
+crossbeam-channel = "0.5"
+tempfile = "3"