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/daterange.go

@@ -0,0 +1,51 @@
+package datetime
+
+import (
+	"log/slog"
+	"time"
+
+	"github.com/egoist/mygo/ui"
+)
+
+type DateRangeOptions struct {
+	Disabled bool
+}
+
+type DateRangeResult struct {
+	Element *ui.Element
+	Changed bool
+}
+
+type rangeKey struct{}
+
+// ranging is the keys' day; tail picks the end next.
+type ranging struct {
+	cur  time.Time
+	tail bool
+}
+
+// DateRange picks a start, then an end; days between dither.
+func DateRange(c *ui.Context, text string, start, end *time.Time, o DateRangeOptions) DateRangeResult {
+	home := ui.Box(c)
+	st := ui.Local(home, rangeKey{}, func() ranging { return ranging{cur: *start} })
+	e, _, picked := month(c, text, &st.cur, o.Disabled, false, func(d time.Time) look {
+		day := time.Date(d.Year(), d.Month(), d.Day(), 12, 0, 0, 0, time.UTC)
+		from := time.Date(start.Year(), start.Month(), start.Day(), 12, 0, 0, 0, time.UTC)
+		to := time.Date(end.Year(), end.Month(), end.Day(), 12, 0, 0, 0, time.UTC)
+		return look{chosen: sameDay(d, *start) || sameDay(d, *end), between: day.Compare(from) > 0 && day.Before(to)}
+	})
+	res := DateRangeResult{Element: e}
+	if picked {
+		switch d := st.cur; {
+		case !st.tail:
+			*start, *end = onDay(*start, d), onDay(*end, d)
+		case d.Before(*start):
+			*start, *end = onDay(*start, d), onDay(*end, *start)
+		default:
+			*end = onDay(*end, d)
+		}
+		st.tail, res.Changed = !st.tail, true
+		slog.Debug("8bit: date range", "label", text, "start", start.Format("2006-01-02"), "end", end.Format("2006-01-02"))
+	}
+	return res
+}