Replay fanout

broadcast_with_replay extends Stream::broadcast with a sliding replay tail.

#![allow(unused)]
fn main() {
use id_effect::{Stream, broadcast_with_replay, run_async};

let src = Stream::from_iterable(1..=100);
let (mut branches, pump) = run_async(
    broadcast_with_replay(src, /* hub */ 64, /* replay tail */ 16, /* branches */ 2),
    (),
)
.await?;

// Run `pump` concurrently with pulls on each branch (same pattern as `broadcast`).
}

• hub_capacity — sliding PubSub ring size.
• replay_len — number of recent items retained in the shared replay buffer (also seeded into each branch buffer as the pump runs).
• Stream::broadcast_replay(cap, branches) — convenience wrapper using replay_len == cap.

Return shape matches broadcast: (Vec<Stream<…>>, pump_effect).