`stdout` and `stderr` are sometimes interleaved

When a `command` produces output on both `stdout` and `stderr`, sometimes both are interleaved.

Small reproducer (with `cargo new t && cd t`):
`src/main.rs`:
```rust
fn main() {
for i in 0..10 {
println!("stdout: {i}");
}
for i in 0..10 {
eprintln!("stderr: {i}");
}
}
```

`bacon run`:
```
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
Running `target/debug/t`
stdout: 0
stderr: 0
stdout: 1
stdout: 2
stdout: 3
stdout: 4
stdout: 5
stdout: 6
stdout: 7
stdout: 8
stdout: 9
stderr: 1
stderr: 2
stderr: 3
stderr: 4
stderr: 5
stderr: 6
stderr: 7
stderr: 8
stderr: 9
```

(This sometimes needs a few tries.)

This is especially noticeable when the program normally produces output on `stdout` and then `dbg!()` is used to produce additional debug output.

---

```console
$ bacon --version
bacon 3.14.0
```

Reproduce the ordering issue with the provided src/main.rs program and bacon run, trying several runs as described. Then locate bacon's command-output handling and trace how stdout and stderr are collected or displayed. Done means output from both streams consistently preserves the command's order without interleaving.

Written by the indexing model from the issue text.