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
-// than the counter-only groups; use a smaller total to keep the bench fast.
+// The handle path does a counter update plus a timestamp `touch` per `add`,
+// 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];