Tryst::UI::TimerHandle

What Session#every and Session#after hand back - one type for both, valid in both phases.

The two things it papers over are real asymmetries underneath. App#after returns an AfterHandle you cancel via App#after_cancel(handle); App#every returns a RepeatingTimer you cancel via timer.cancel - two types, two spellings. And a timer declared inside the build block has no live Tcl timer to hand back at all yet, since nothing has registered with the interpreter at that point.

So this holds a cancel action instead of a timer. Before realize there is none, and #cancel just marks the handle - Session's flush skips it, and it never registers. After realize (or if declared post-realize) the action is the real cancel for whichever kind of timer it turned out to be. Either way the caller writes handle.cancel and it does the right thing, so a tick loop can be declared right alongside the UI it drives and still be cancellable from anywhere.