## Why
Failed shell snapshot captures could leave processes launched by shell startup files running after the capture ended.
## What changed
Run each Unix shell snapshot capture in its own process group and kill that group on failure, timeout, or cancellation. Keep the capture leader unreaped until output and exit status are validated so its process group ID cannot be reused during cleanup. Preserve startup helpers after a successful capture.
## Testing
Add regression coverage for startup child cleanup after nonzero exits, invalid output, cancellation, and timeout, alongside fallback and retry behavior. Verify that successful captures preserve delayed startup output and background services and reuse the cached snapshot.
GitOrigin-RevId: a56ef5d8615325952bdc8bb78727de1559e7e9c7
