Replace Windows SuspendThread/SetThreadContext with atomic-flag preemption

Remove assembly stubs, CONTEXT constants, SuspendThread/GetThreadContext/
SetThreadContext/ResumeThread, and DuplicateHandle. Instead, the monitor
thread sets an AtomicBool flag per coroutine, and the EventLoop thread
checks it via Monitor::check_preempt() in the syscall facade.

On Unix: unchanged — SIGURG asynchronously interrupts the thread.
On Windows: cooperative — flag checked at syscall boundaries (every
hooked Sleep/recv/send/etc triggers a check). Pure CPU-bound loops
without syscalls cannot be preempted on Windows.

Also gate coroutine_preemptive test (which uses `loop {}`) to Unix only,
since a pure CPU loop can't be cooperatively preempted.

Agent-Logs-Url: https://github.com/acl-dev/open-coroutine/sessions/b1d08a87-9c5f-425c-b298-4835c4bb2bce

Co-authored-by: loongs-zhang <38336731+loongs-zhang@users.noreply.github.com>

Author: Copilot