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/connector/manual.rs

@@ -2,10 +2,11 @@ use std::{
     collections::BTreeSet,
     future::Future,
     sync::{Arc, Weak},
-    time::{Duration, Instant},
+    time::Duration,
 };
 
 use dashmap::DashSet;
+use hotpath::instant::Instant;
 use tokio::{sync::mpsc, task::JoinSet, time::timeout};
 
 use crate::{