fix(native): make Windows instance shutdown cancellable

[Milky2018]

Background

The Windows native smoke test can occasionally hang until the CI timeout while destroying the primary application instance. Normal runs finish in well under a second, so this is a lifecycle race rather than expected test duration.

The Windows single-instance listener used synchronous named-pipe operations. Shutdown attempted to interrupt the listener with CancelSynchronousIo and then connected a temporary client to wake ConnectNamedPipe. There is a race where cancellation finds no pending I/O and the temporary connection happens before the listener creates its next pipe. The listener can then enter a new blocking ConnectNamedPipe call, while destruction waits indefinitely for that thread.

Changes

• Open the server-side named pipe with FILE_FLAG_OVERLAPPED.
• Make server-side connect, read, and write operations wait on both I/O completion and the instance stop_event.
• Cancel and drain the exact pending overlapped operation when shutdown wins the wait.
• Replace the blocking FlushFileBuffers step with an ACK receipt handshake so shutdown remains cancellable without risking premature disconnect.
• Remove the racy CancelSynchronousIo and temporary-client wakeup path.

The public ABI and activation JSON format are unchanged.

Validation

• moon fmt
• moon info
• cmake --build native/build-engine --config Debug
• ctest --test-dir native/build-engine -C Debug --output-on-failure (6/6 passed)

The Windows-specific implementation cannot be executed on the current macOS host and requires Windows CI validation.