Add Criterion multi-threaded runtime benchmarks and a live dashboard load example.
- benches/mt_benchmarks.rs: add a Criterion suite with `mt/single_actor`, `mt/multi_actor`, `mt/ring`, and `mt/spawn` benchmarks over 2-4 threads, using `iter_custom` and a `wait_for_n_done` helper for deterministic completion
- crates/runtime-dashboard/examples/bench_dashboard.rs: add an example driving sink/ring/spawner load scenarios through `start_dashboard`/`DashboardConfig` for sustained cross-worker visualization
- Cargo.toml: register the `mt_benchmarks` Criterion bench target with `harness = false`
Signed-off-by: Zachery Aaron Shores-Chmielewski <zacheryasc@gmail.com>
