fix(win32): game-loop pacing so the event loop isn't starved

[KoalaHao]

ReadConsoleInputW is a blocking native call. In the old code, the Dart event loop had no chance to run between input events, so periodic redraws (cursor blink, animations, status ticks) never fired. The framework appeared frozen on Windows until an event arrived.

Switched to game-loop pacing: capture a 16ms deadline at the start of each pass, drain the console queue, then sleep only for the remaining time. A slow pass (big redraw, burst of events) skips the sleep entirely and runs the next pass immediately, so we never double-charge a 16ms wait on top of work and the input loop stays responsive regardless of workload.

GetNumberOfConsoleInputEvents is a non-blocking peek, used to size the drain loop. Subsequent ReadConsoleInputW calls return immediately when events are pending, so the isolate is never blocked waiting for input. The drain loop also means a burst of events (fast typing, fast mouse) is processed in a single tick instead of throttled to one record per poll.

16ms ~= 60Hz, matching the Linux path's responsiveness. Idle CPU is ~0% because Future.delayed is timer-backed, not busy-polled.