Address review on the bash stdin/env seam PR: the seam spawned stdin as a
`'pipe'` for EVERY call, closing it empty when no stdin was supplied. That is
NOT observationally equivalent to the pre-seam `'ignore'` default — node's
spawn pipe is an AF_UNIX socket, so `test -c /dev/stdin` (and any fd-0 type
probe) flipped for every model-driven bash call, even though the code claimed
the no-stdin path was unchanged.
Spawn stdin as `'pipe'` only when the caller supplies bytes; otherwise
`'ignore'` (fd 0 → /dev/null), the exact prior default. A literal `stdio`
tuple per branch preserves the typed `spawn` overload's non-null stdout/stderr.
Regression test asserts fd 0 is a char device with no stdin and a socket when
supplied — proven red on the always-pipe code.
Tianyi Cui
40488e29e5