A script whose descendant outlives npm never finishes: the app waits on close while an orphan holds the pipe

Description

When a script's process tree is broken from outside (on Windows, closing a console window the Gutenberg build opened, #497), npm exits but a descendant (wp-build) survives holding the stdout pipe. The app marks a script finished on the child's close event, which does not fire while any descendant keeps the pipe open, so the checklist stays on IN PROGRESS, the button stays busy, and the terminal keeps receiving the orphan's output long after npm verbose exit 1. Ctrl+C in the terminal (kill by run id) is the only way out; a contributor does not know that.

Fix idea. Mirror #479 on the natural-exit path: on exit, if close has not followed within a few seconds, force the tree by pid (killTreeByPid) and finish with the exit code npm gave. The orphan's later output is dropped. Same shape for installs and the Playground server.

Step-by-step reproduction instructions

1. Windows, Gutenberg site, Run full build.
2. Close one of the console windows the build opens (#497).

Expected: the step turns FAILED when npm exits. Actual: IN PROGRESS for as long as the orphan runs (tens of minutes on a slow VM).

Operating system

Windows 11 (UTM VM). The close-vs-exit logic is shared, so a POSIX orphan holding the pipe would do the same.

App version

Trunk at 4c53900 (v1.2.0-beta.1 pass).

Start by tracing the shared close-versus-exit handling described in the issue, including the natural-exit path mirrored from #479. Check how killTreeByPid is used for the main script, installs, and the Playground server. Done means an npm exit marks the step failed promptly, terminates the orphaned tree after the timeout, and drops later output.