Module capture 

--capture mode: replace each child’s stdout with a piped handle, buffer to completion, emit captured chunks in argv order after all children exit (FR-017 + AD-007).

Without --capture (the default), children inherit the parent’s stdout and their outputs interleave nondeterministically. With --capture, the parent guarantees argv-ordered emission — useful for tests and scripts that need reproducible output (US5).

Clarification Q2 + Q6 from spec.md:

• Q2: at spawn time, each child’s stdout is replaced with Stdio::piped().
• Q6: empty-stdout children emit nothing (no separator inserted).

Functions

run_with_capture

Drive the fan-out + capture: feed reader to every child’s stdin, then read every child’s stdout into a per-argv-position buffer, then wait + collect statuses, then emit captured bytes in argv order to out.

spawn_all_piped

Spawn all commands with piped stdout (for capture mode). Returns the spawned children in argv order. On any spawn failure, kills already-spawned children and returns the io error.