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.

Cargo.lock

@@ -2362,6 +2362,7 @@ dependencies = [
  "prost-reflect",
  "prost-reflect-build",
  "prost-wkt-types",
+ "quanta",
  "quinn",
  "quinn-proto",
  "quote",