process pool: inbound lines as String assume UTF-8

Came up while planning the optative-process-pool extraction.

`BufRead::lines()` yields `Result` and errors on non-UTF-8 bytes. The current reader thread would silently break and stop forwarding output. Anything emitting raw bytes or running under a broken locale would hit this.

`Vec` as the payload (with `read_until(b'\n', _)`) is the safer default. Users who want a String can do `String::from_utf8_lossy` at the call site.

Drafted by Claude (AI assistant) during a design review.