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/Cargo.toml

@@ -53,6 +53,7 @@ chrono = { version = "0.4.37", features = ["serde"] }
 
 guarden = "0.2"
 hotpath = { version = "0.18", default-features = false, optional = true }
+quanta = "0.12"
 
 delegate = "0.13.5"