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/lib.rs

@@ -14,6 +14,21 @@ extern crate self as hotpath;
 #[cfg(not(feature = "hotpath"))]
 mod hotpath_off;
 
+// When the `hotpath` feature is off, expose a local `instant` module backed by
+// `quanta::Instant` so call sites can uniformly write `use hotpath::instant::Instant;`
+// regardless of whether the feature is enabled. With the feature on, the real
+// `hotpath` crate provides the same path (also `quanta::Instant` on Linux), so
+// the two modes resolve to the identical type.
+#[cfg(not(feature = "hotpath"))]
+pub mod instant {
+    pub type Instant = quanta::Instant;
+}
+
+// Re-export `Instant` at the crate root so public APIs that expose it
+// (e.g. `Route::get_peer_info_last_update_time`) reference a deliberate
+// public type rather than leaking an inaccessible one.
+pub use hotpath::instant::Instant;
+
 mod arch;
 mod gateway;
 pub mod instance;