IOCP: missing Windows equivalents of the revision-2 process and file contracts #10

The IOCP backend's Windows contract coverage is still missing equivalents that docs/BACKEND_REVISION_2.md asks for:

• closing a live child with a pending exit watch (Cancelled → Closed, child actually reaped),
• dropping the loop while children are live,
• argv quoting, environment and cwd round-trip (including quotes, trailing backslashes and non-ASCII),
• worker-file FIFO ordering with drop quiescence.

Exit and signal delivery also have no Windows allocation gate, because signal_exit_and_external_notification_delivery_allocate_nothing is cfg(unix) (it needs libc). PR #5 added windows_child_watch_cancel_exit_and_close_allocate_nothing_after_setup, which covers exit watches; console control events are still ungated.

Found in the PR #5 integrator review (item 8).