bench: support mimalloc/jemalloc allocator in cpu_hotspot_ring example

Add #[global_allocator] behind feature flags so the bench can test
different allocators. Previously the example used glibc malloc by
default (easytier-core.rs sets jemalloc/mimalloc only for the bin
target, not examples).

Benchmark (4 threads, 1400B, 15s, clone mode):
  glibc:    246K pps, 3.13us/pkt
  jemalloc: 246K pps, 3.21us/pkt
  mimalloc: 242K pps, 3.25us/pkt

All within noise. Single-threaded clone has low malloc contention;
~1500B small allocs are served efficiently by all tcaches.

easytier/examples/cpu_hotspot_ring.rs

@@ -15,6 +15,14 @@
 //! Then in another terminal:
 //!   hotpath console
 
+#[cfg(feature = "mimalloc")]
+#[global_allocator]
+static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;
+
+#[cfg(feature = "jemalloc")]
+#[global_allocator]
+static ALLOC: jemallocator::Jemalloc = jemallocator::Jemalloc;
+
 use std::net::IpAddr;
 use std::time::{Duration, Instant};