fix: stop pack:verify leaving its test server running on Windows

[ChrystianSchutz]

On Windows the installed bin is a .cmd shim spawned through cmd.exe (shell: true), so child.kill() in stopChild only killed the shell. The ThreadShelf server it started kept running with the inherited stdout pipe open, so npm run pack:verify | tail never finished even after all checks passed.

stopChild now uses taskkill /T /F on Windows to end the whole process tree. Other platforms are unchanged.

Testing

• npm run pack:verify 2>&1 | tail -3 on Windows: 20/20 checks, exits in ~22 s, and no server stays listening on ports 3900–3989. It previously hung until the server was killed by hand.