perf(core): use quanta::Instant for hot-path timing (#2384)

Replace std::time::Instant with quanta::Instant on per-packet, per-RPC,
and per-session paths. TSC-based, ~5ns vs ~25ns per now() call.

Reuses the existing `extern crate self as hotpath` alias so
`use hotpath::instant::Instant;` resolves to the same quanta type with
or without the hotpath feature. Leaves tokio::time::Instant and
smoltcp::time::Instant untouched.

easytier/src/common/stats_manager.rs

@@ -1,9 +1,10 @@
 use dashmap::DashMap;
+use hotpath::instant::Instant;
 use serde::{Deserialize, Serialize};
 use std::cell::UnsafeCell;
 use std::fmt;
 use std::sync::Arc;
-use std::time::{Duration, Instant};
+use std::time::Duration;
 use tokio::time::interval;
 use tokio_util::task::AbortOnDropHandle;