fix: cold benchmark cache-busting with PID prefix and flush

Re-runs of --vs-unbound-cold were hitting stale cache entries from
prior runs. The static COUNTER reset to 0 each process, generating
the same c0.example.com subdomains. With the 1-hour stale window,
entries from 10 minutes ago served as stale hits.

Fix: prefix with PID (r{pid}-c{n}.domain) and flush Numa's cache
before cold benchmarks.

benches/recursive_compare.rs

@@ -403,6 +403,8 @@ fn run_server_comparison(
 ) {
     use std::sync::atomic::{AtomicU64, Ordering};
     static COUNTER: AtomicU64 = AtomicU64::new(0);
+    // Unique prefix per process so re-runs don't hit stale cache entries
+    let run_id = std::process::id();
 
     let numa_addr: SocketAddr = NUMA_BENCH.parse().unwrap();
     let other: SocketAddr = other_addr.parse().unwrap();
@@ -414,6 +416,10 @@ fn run_server_comparison(
         }
     }
 
+    if cold {
+        flush_cache(); // flush Numa's record cache
+    }
+
     println!("Warming up...");
     for _ in 0..5 {
         let _ = rt.block_on(query_udp(numa_addr, "example.com"));
@@ -433,7 +439,12 @@ fn run_server_comparison(
         other_name,
         &|domain| {
             let d = if cold {
-                format!("c{}.{}", COUNTER.fetch_add(1, Ordering::Relaxed), domain)
+                format!(
+                    "r{}-c{}.{}",
+                    run_id,
+                    COUNTER.fetch_add(1, Ordering::Relaxed),
+                    domain
+                )
             } else {
                 domain.to_string()
             };
@@ -443,7 +454,12 @@ fn run_server_comparison(
         },
         &|domain| {
             let d = if cold {
-                format!("c{}.{}", COUNTER.fetch_add(1, Ordering::Relaxed), domain)
+                format!(
+                    "r{}-c{}.{}",
+                    run_id,
+                    COUNTER.fetch_add(1, Ordering::Relaxed),
+                    domain
+                )
             } else {
                 domain.to_string()
             };