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.

datetime/countdown.go

@@ -0,0 +1,55 @@
+package datetime
+
+import (
+	"log/slog"
+	"time"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type CountdownOptions struct{}
+
+type CountdownResult struct {
+	Element *ui.Element
+	// Done is true in the frame time runs out.
+	Done bool
+}
+
+type countKey struct{}
+
+// counting is the deadline counted to, and whether it passed.
+type counting struct {
+	to   time.Time
+	over bool
+}
+
+// Countdown shows time left to deadline, Bad near the end.
+func Countdown(c *ui.Context, text string, deadline time.Time, o CountdownOptions) CountdownResult {
+	k := pixel.K(c)
+	e := ui.Box(c).Role(ui.RoleStatus).Label(text)
+	st := ui.Local(e, countKey{}, func() counting { return counting{} })
+	if !st.to.Equal(deadline) {
+		*st = counting{to: deadline}
+	}
+	left := max(0, deadline.Sub(k.Now()))
+	// Seconds round up: zero shows only at the end.
+	shown := (left + time.Second - 1).Truncate(time.Second)
+	res := CountdownResult{Element: e}
+	if left == 0 && !st.over {
+		st.over, res.Done = true, true
+		slog.Debug("8bit: countdown done", "label", text)
+	}
+	if left > 0 {
+		k.At(c, deadline.Add(-shown+time.Second))
+	}
+	s, ink := clock(shown), pixel.Ink
+	if left < 10*time.Second {
+		ink = pixel.Bad
+	}
+	e.Description(s).Size(k.D(pixel.Measure(s)*2), k.D(pixel.LineHeight*2)).Draw(func(p *ui.Painter, r ui.Rect) {
+		k.Canvas(p, r).Text(0, 0, s, ink, 2)
+	})
+	return res
+}