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/datetime_test.go

@@ -0,0 +1,91 @@
+package datetime_test
+
+import (
+	"image"
+	"testing"
+	"time"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type clock struct{ t time.Time }
+
+func (c *clock) now() time.Time { return c.t }
+
+// newClock reads Sunday 11 October 2026, 09:30 UTC.
+func newClock() *clock { return &clock{time.Date(2026, 10, 11, 9, 30, 0, 0, time.UTC)} }
+
+// view builds on a fake clock; inert veils every control.
+func view(w, h int, clk *clock, inert bool, build func(c *ui.Context)) *ui.Tester {
+	return ui.NewTester(func(c *ui.Context) {
+		pixel.Use(c, pixel.Settings{Palette: pixel.SpringDay, Now: clk.now, Inert: inert})
+		build(c)
+	}, w, h)
+}
+
+func find(t *testing.T, tt *ui.Tester, label string) ui.Rect {
+	t.Helper()
+	r, ok := tt.Find(label)
+	if !ok {
+		t.Fatalf("no %q in %q", label, tt.Texts())
+	}
+	return r
+}
+
+func click(t *testing.T, tt *ui.Tester, label string) {
+	t.Helper()
+	if err := tt.Click(label); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func shows(tt *ui.Tester, label string) bool {
+	_, ok := tt.Find(label)
+	return ok
+}
+
+// slot is the palette slot of pixel x, y.
+func slot(t *testing.T, img *image.RGBA, x, y int) pixel.Slot {
+	t.Helper()
+	c := img.RGBAAt(x, y)
+	for s, pc := range pixel.SpringDay.Colors {
+		if pc.R == c.R && pc.G == c.G && pc.B == c.B {
+			return pixel.Slot(s)
+		}
+	}
+	t.Fatalf("pixel %d,%d is %v, off the palette", x, y, c)
+	return 0
+}
+
+// keeps fails on off-palette colors or split dots.
+func keeps(t *testing.T, img *image.RGBA) {
+	t.Helper()
+	b := img.Bounds()
+	for y := 0; y+1 < b.Dy(); y += 2 {
+		for x := 0; x+1 < b.Dx(); x += 2 {
+			slot(t, img, x, y)
+			c := img.RGBAAt(x, y)
+			if img.RGBAAt(x+1, y) != c || img.RGBAAt(x, y+1) != c || img.RGBAAt(x+1, y+1) != c {
+				t.Fatalf("dot at %d,%d is split", x/2, y/2)
+			}
+		}
+	}
+}
+
+// count tallies dots of slot s in r.
+func count(t *testing.T, img *image.RGBA, r ui.Rect, s pixel.Slot) int {
+	t.Helper()
+	n := 0
+	for y := int(r.Y); y < int(r.Y+r.H); y += 2 {
+		for x := int(r.X); x < int(r.X+r.W); x += 2 {
+			if slot(t, img, x, y) == s {
+				n++
+			}
+		}
+	}
+	return n
+}
+
+func day(t time.Time) string { return t.Format("2006-01-02") }