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/calendar.go

@@ -0,0 +1,29 @@
+package datetime
+
+import (
+	"log/slog"
+	"time"
+
+	"github.com/egoist/mygo/ui"
+)
+
+type CalendarOptions struct {
+	Disabled bool
+}
+
+type CalendarResult struct {
+	Element *ui.Element
+	// Picked is a click, Enter or Space on a day.
+	Changed, Picked bool
+}
+
+// Calendar moves *date through a month grid; today is marked.
+func Calendar(c *ui.Context, text string, date *time.Time, o CalendarOptions) CalendarResult {
+	old := *date
+	e, _, picked := month(c, text, date, o.Disabled, false, func(d time.Time) look { return look{chosen: sameDay(d, *date)} })
+	res := CalendarResult{Element: e, Changed: !date.Equal(old), Picked: picked}
+	if res.Changed || res.Picked {
+		slog.Debug("8bit: calendar", "label", text, "date", date.Format("2006-01-02"), "picked", picked)
+	}
+	return res
+}