Public API for bounded supervised work.

Task references are stable binaries. Work is owned by adk_task_worker under adk_task_sup, not by the process that starts or awaits it. A task has exactly one outcome:

{completed, Value} {failed, Reason} {timed_out, deadline_exceeded} {cancelled, Reason}

The deadline option is an absolute Erlang monotonic millisecond value. A relative timeout is converted to that absolute deadline once, before the supervised task starts, so queue/startup time consumes the same budget.