process pool: unbounded mpsc channels give no backpressure

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

Inbound `mpsc::Sender` and per-child outbound `mpsc::Sender>` are unbounded. A slow consumer of inbound stdout, or a fast producer of outbound stdin, will grow memory without bound.

`std::sync::mpsc::sync_channel(n)` (or `crossbeam-channel`) gives us backpressure for free. The only design choice is the default `n` and whether it's configurable per direction.

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