fix: wire up timer lifecycle in add/update/remove handlers - #570

Thanks for this, @nhwilly — you spotted a real bug.

AddTimer / UpdateTimer still store a bare new Timer(duration) without Elapsed, AutoReset = false, or Start(). Only Initialize wires those. That is exactly kanban 064 (Fix timer actions never wiring created timers).

We are not merging this PR:

• The same work is already tracked as 064 (still to-do; not landed on master yet).
• The branch is based on the pre-080 handler shape (Task Handle vs ValueTask, older mediator types), so it would not apply cleanly on current master.

When we pick up 064 we will extract a shared create-and-start helper and use it from Initialize / AddTimer / UpdateTimer, and stop+dispose on replace/remove — the same approach you took here.

Closing in favor of 064. Appreciate you bringing it to our attention.