Popen.kill() only terminated the direct child, so runner-style commands
(npx -> node -> server) leaked the real server process on every Windows
spawn test. taskkill /PID <pid> /T /F walks the descendant tree. Also
sets CREATE_NO_WINDOW on the spawned test process so the windowed exe
doesn't flash a console per test.
Closes#13
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>