docs(stats): fix misleading/stale comments and rename test_counter

- GC test: cutoff is in the future, so every metric is stale by timestamp
  (not "nothing is stale"); only live handles retain.
- GC loop: drop the inaccurate "no Instant alloc" rationale.
- bench: the handle path no longer calls Instant::now() (it uses fastant);
  reword the HANDLE_TOTAL_WORK rationale.
- rename test_unsafe_counter -> test_counter to match the type rename.

easytier/benches/counter_contention.rs

@@ -31,8 +31,9 @@ use parking_lot::Mutex;
 
 const COUNTER_SHARDS: usize = 16;
 const TOTAL_WORK: u64 = 8_000_000;
-// The handle path calls `Instant::now()` per `add`, so it is far heavier per op
+// The handle path does a counter update plus a timestamp `touch` per `add`,
-// than the counter-only groups; use a smaller total to keep the bench fast.
+// so it is heavier per op than the counter-only groups; use a smaller total to
+// keep the bench fast.
 const HANDLE_TOTAL_WORK: u64 = 2_000_000;
 const TASK_COUNTS: &[usize] = &[1, 2, 4, 8, 16, 32];
 

easytier/src/common/stats_manager.rs

@@ -578,7 +578,8 @@ impl StatsManager {
                 interval.tick().await;
 
                 // Drop metrics untouched for 180s and with no live handles.
-                // Compare in the millis-since-base domain; no Instant alloc.
+                // Compare in the millis-since-base domain so neither the hot
+                // path nor GC reconstructs an `Instant` or locks.
                 let cutoff_millis = now_millis().saturating_sub(180_000);
 
                 let Some(counters) = counters_clone.upgrade() else {
@@ -756,7 +757,7 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn test_unsafe_counter() {
+    async fn test_counter() {
         let counter = Counter::new();
 
         assert_eq!(counter.get(), 0);
@@ -908,8 +909,8 @@ mod tests {
         let counter = stats.get_simple_counter(MetricName::TrafficBytesForwarded);
         counter.set(1);
 
-        // Cutoff 1s in the future, so nothing is stale by timestamp; only live
+        // Cutoff 1s in the future, so every metric is stale by timestamp; only
-        // handles keep a metric.
+        // a live handle keeps a metric.
         let cutoff_millis = now_millis() + 1_000;
         stats
             .counters