part-bench: cool down for 4s between runs

part-bench pins the rayon threads to CPU cores, so that runs at lower
thread counts are not affected by how memory is laid out.  On strong
scaling runs, this can cause overheating since the first cores are
working the most, while the later ones start fresh.

tools/src/bin/part-bench/main.rs

@@ -6,6 +6,8 @@ use mesh_io::weight;
 use std::env;
 use std::fs;
 use std::io;
+use std::thread::sleep;
+use std::time::Duration;
 
 const USAGE: &str = "Usage: part-bench [options]";
 
@@ -135,11 +137,16 @@ fn main_d<const D: usize>(
         let max_threads = rayon::current_num_threads();
         let mut g = c.benchmark_group(benchmark_name);
         let mut thread_count = 1;
-        while thread_count <= max_threads {
+        loop {
             let pool = build_pool(thread_count);
             let benchmark_name = format!("threads={thread_count}");
             g.bench_function(&benchmark_name, |b| pool.install(|| b.iter(&mut benchmark)));
             thread_count *= 2;
+            if max_threads < thread_count {
+                break;
+            }
+            println!("Waiting 4s for CPUs to cool down...");
+            sleep(Duration::from_secs(4));
         }
     } else {
         c.bench_function(&benchmark_name, |b| b.iter(&mut benchmark));