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/tunnel/wireguard.rs

@@ -6,6 +6,8 @@ use std::{
     time::Duration,
 };
 
+use hotpath::instant::Instant;
+
 use super::{
     FromUrl, IpVersion, Tunnel, TunnelError, TunnelInfo, TunnelListener, TunnelUrl, ZCPacketSink,
     ZCPacketStream,
@@ -346,7 +348,7 @@ struct WgPeer {
     data: Option<WgPeerData>,
     tasks: JoinSet<()>,
 
-    access_time: AtomicCell<std::time::Instant>,
+    access_time: AtomicCell<Instant>,
 }
 
 impl WgPeer {
@@ -369,7 +371,7 @@ impl WgPeer {
             data: None,
             tasks: JoinSet::new(),
 
-            access_time: AtomicCell::new(std::time::Instant::now()),
+            access_time: AtomicCell::new(Instant::now()),
         }
     }
 
@@ -385,7 +387,7 @@ impl WgPeer {
     }
 
     async fn handle_packet_from_peer(&self, packet: &[u8]) {
-        self.access_time.store(std::time::Instant::now());
+        self.access_time.store(Instant::now());
         tracing::trace!("Received {} bytes from peer", packet.len());
         let data = self.data.as_ref().unwrap();
         // TODO: improve this