StartCountdownTimer

Starts a countdown in a coroutine launched from the running event's context, ticking down from timerData's initial toward zero in step-sized decrements, waiting one step between ticks — built with milliseconds or seconds. The event returns as soon as the countdown is launched, so the chain continues while the timer runs in the background; it stops on its own when the countdown ends. To stop it early, launch it from inside a RunCancellableEvents and cancel that id with CancelEvents. Does not consume incomingData. Dispatches onCountdownTimerTick once per tick (carrying the remaining amount), and onTimeFinish once after the last tick — neither success nor failure is reported.

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