//! Opt-in feature modules. Each entry is gated by its own Cargo
//! feature; the base `HdrHistogram` in `lib.rs` stays zero-dep + std-only.
// `dual-recorder` builds on `concurrent-writes` because the active
// histogram must accept concurrent producer writes while the consumer
// hot-swaps. Cargo enforces the implies via `dual-recorder =
// ["concurrent-writes"]`.