Add datetime components 196-215; Kit.At; inert stepper arrows

Calendars, date and time inputs, pickers, clocks, timers, a sun dial,
a season wheel, an agenda and a day schedule. Kit.At rebuilds at a
moment (relative times, midnight). Stepper and NumberInput arrows stop
under Inert. DateInput parks a popup focus as its own.

AGENTS.md

@@ -55,7 +55,7 @@ Imports point down only. Same-tier imports are forbidden. `internal/archtest` en
 
 - Signatures and results: PRD §5.1. Zero options give the default look. Sizes in options are dots.
 - `k := pixel.K(c)` once per component; layout sizes through `k.D(n)`. Integer options default through `pixel.Or`.
-- Custom-drawn controls end their Draw with `g.Finish(st)`. Grids of cells moved by arrows use `pixel.Pad`; one-shot timing steps with `k.Since`.
+- Custom-drawn controls end their Draw with `g.Finish(st)`. Grids of cells moved by arrows use `pixel.Pad`; one-shot timing steps with `k.Since`; a view that changes at a known moment asks `k.At`.
 - A one-shot effect inside a lasting view starts when its cue changes (a `Cue` option read through `k.Cue`, or a value that rises), never on its first frame. A state screen (`ErrorState`, `SuccessState`) starts when it appears. Decorative motion jumps to its end under Reduce Motion; countdowns and loaders keep time.
 - Containers take content as a final `func()`; interactive results are `NameResult`.
 - A Clicked, Shortcut, Changed or drag during the build makes MyGo build the frame again, so what they change shows at once. State changed any other way after its element was built (`Pressed`, `Edit.Changed`, a drag's release) is read at paint time, or set on the element after its children.