[codex] honor child_process numeric stdio fds

[andrewtdiz]

Summary

This adds live child_process support for numeric fd entries in spawn() and fork() stdio arrays. Valid fd entries are routed to duplicated parent file descriptors, while the JS-visible stdin/stdout/stderr properties and child.stdio[] slots match Node by exposing null for those entries.

The change also parses array stdio forms before string forms so mixed arrays such as ["ignore", outFd, errFd] are not accidentally interpreted as strings, and keeps fork IPC behavior intact for ["ignore", outFd, errFd, "ipc"].

Non-goals for this slice: custom stream handles, invalid-fd validation/error shape, sync stdio fd forms, numeric stdin coverage, AbortSignal/killSignal timeout behavior, uid/gid, detached/platform flags, and broad option validation.

Validation

• npm exec --yes --package=node@26 -- node --experimental-strip-types test-parity/node-suite/child_process/async/stdio-fd.ts
• PERRY_NO_AUTO_OPTIMIZE=1 npm exec --yes --package=node@26 -- bash -lc './run_parity_tests.sh --suite node-suite --module child_process --filter stdio-fd'
• PERRY_NO_AUTO_OPTIMIZE=1 npm exec --yes --package=node@26 -- bash -lc './run_parity_tests.sh --suite node-suite --module child_process --filter stdio-ignore'
• PERRY_NO_AUTO_OPTIMIZE=1 npm exec --yes --package=node@26 -- bash -lc './run_parity_tests.sh --suite node-suite --module child_process --filter stdio-inherit'
• PERRY_NO_AUTO_OPTIMIZE=1 npm exec --yes --package=node@26 -- bash -lc './run_parity_tests.sh --suite node-suite --module child_process' (18/18)
• cargo fmt --all -- --check
• git diff --check
• ./scripts/check_file_size.sh
• CARGO_TARGET_DIR=/tmp/perry-child-stdio-fd-check cargo check -p perry-runtime (passes with existing warnings)