Windows: handle orphaned rv.exe task (edge case)

[case]

This could come up someday: When a parent wrapper process (rvw) is killed (e.g. by Task Scheduler timeout, taskkill), the child rv.exe keeps running with no parent. uv's solution is clean (🙌): create a Windows Job Object with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE, which automatically terminates child tasks when the parent task exits.