[codex] honor child_process detached process groups

[andrewtdiz]

Summary

• add a child_process parity fixture for detached: true process-group behavior across spawn, spawnSync, and fork
• wire options.detached === true into shared spawn/spawnSync command construction
• apply the same detached setup to the fork command path before launching IPC-enabled children

Root Cause

Perry already exposed the documented child_process option surface, but the live and sync command builders ignored options.detached. On Unix, Node makes a detached child the leader of a new session and process group, so the child shell's process group ID should match its PID. Perry left children in the parent process group.

Validation

• npm exec --yes --package=node@26 -- node --experimental-strip-types test-parity/node-suite/child_process/async/detached-process-group.ts
• Baseline check before the fix: focused parity reported Node.js: spawn detached same pgid: true and Perry: spawn detached same pgid: false
• npm exec --yes --package=node@26 -- bash -lc './run_parity_tests.sh --suite node-suite --module child_process --filter detached-process-group' (1 pass, 0 compile failures)
• npm exec --yes --package=node@26 -- bash -lc './run_parity_tests.sh --suite node-suite --module child_process' (18 pass, 0 compile failures)
• cargo fmt --all -- --check
• git diff --check
• ./scripts/check_file_size.sh

Covers the detached option slice of #2555.