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  the handler-active-but-state-unpublished race window)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/runner.rs

@@ -0,0 +1,59 @@
+use crate::metrics::MetricsCollector;
+use crate::types::BenchmarkResult;
+use std::path::PathBuf;
+
+pub struct BenchConfig {
+    pub test_file: PathBuf,
+    pub quick_mode: bool,
+    pub suites: Option<Vec<String>>,
+}
+
+fn warn_reset_hwm() {
+    if let Err(e) = MetricsCollector::reset_vm_hwm() {
+        if e.kind() == std::io::ErrorKind::PermissionDenied {
+            eprintln!("WARNING: VmHWM reset requires root: {e}");
+        }
+    }
+}
+
+pub fn run_all(config: &BenchConfig) -> Vec<BenchmarkResult> {
+    let mut results = Vec::new();
+
+    let should_run = |name: &str| -> bool {
+        match &config.suites {
+            Some(suites) => suites.iter().any(|s| s == name),
+            None => true,
+        }
+    };
+
+    if should_run("startup") {
+        warn_reset_hwm();
+        results.extend(crate::suites::startup::run(config));
+    }
+    if should_run("render") {
+        warn_reset_hwm();
+        results.extend(crate::suites::render::run(config));
+    }
+    if should_run("jump") {
+        warn_reset_hwm();
+        results.extend(crate::suites::jump::run(config));
+    }
+    if should_run("memory") {
+        warn_reset_hwm();
+        results.extend(crate::suites::memory::run(config));
+    }
+    if should_run("growth") {
+        warn_reset_hwm();
+        results.extend(crate::suites::growth::run(config));
+    }
+    if should_run("rotation") {
+        warn_reset_hwm();
+        results.extend(crate::suites::rotation::run(config));
+    }
+    if should_run("concurrent") {
+        warn_reset_hwm();
+        results.extend(crate::suites::concurrent::run(config));
+    }
+
+    results
+}