PTY session primitives shared by ArcBox guest agents.
Both interactive-exec implementations — the sandbox microVM's vm-agent
and the machine-level session in arcbox-agent — need the same subtle,
security-relevant steps: allocating a sized PTY, wiring the slave as the
child's controlling terminal, and dropping privileges in the one order
that works. This crate is the single home for those steps; everything the
consumers legitimately differ on (process reaping, sync vs async pumps,
wire framing) stays with them.
Linux-only: on other targets the crate compiles to nothing so host-side workspace builds stay unaffected.