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/gateway/socks5.rs

@@ -5,10 +5,11 @@ use std::{
         Arc, Weak,
         atomic::{AtomicBool, AtomicUsize, Ordering},
     },
-    time::{Duration, Instant},
+    time::Duration,
 };
 
 use crossbeam::atomic::AtomicCell;
+use hotpath::instant::Instant;
 #[cfg(feature = "kcp")]
 use kcp_sys::{endpoint::KcpEndpoint, stream::KcpStream};
 use tokio_util::sync::{CancellationToken, DropGuard};