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/peers/peer_conn_ping.rs

@@ -6,6 +6,7 @@ use std::{
     time::Duration,
 };
 
+use hotpath::instant::Instant;
 use rand::{Rng, thread_rng};
 use tokio::{
     sync::broadcast,
@@ -177,7 +178,7 @@ impl PeerConnPinger {
         sink.send(req).await?;
         control_metrics.record_tx(req_len);
 
-        let now = std::time::Instant::now();
+        let now = Instant::now();
         // wait until we get a pong packet in ctrl_resp_receiver
         let resp = timeout(Duration::from_secs(2), async {
             loop {