Add media components 346-365; input.SlideBase, guarded Pad

An audio player with transport, scrubber and volume; an equalizer and
a visualizer; a playlist; a cassette, cartridge and record that turn
while they play and hold where they stop; piano keys, a step sequencer
and a tracker you can type into; a waveform, a 16:9 video frame, an
image compare, a gallery, a filmstrip, a record button and an FM tuner.

input exports SlideBase, the undrawn rail its sliders share; the
gallery treats exported Bases as building blocks, not components.
pixel's micro font gains '#' for notes like C#4. pixel.Pad and the
slide rail read input only while their base is enabled, so Inert or a
fresh Disabled stops a drag or a key already sent. AGENTS records why
a drawn container keeps the DISABLED description: MyGo paints a
disabled subtree at half opacity.

media/visualizer.go

@@ -0,0 +1,49 @@
+package media
+
+import (
+	"fmt"
+	"math"
+	"strings"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type VisualizerOptions struct {
+	// Height is in dots; zero means 32.
+	Height int
+}
+
+// Visualizer mirrors each level, 0 to 1, about the middle.
+func Visualizer(c *ui.Context, text string, levels []float64, o VisualizerOptions) *ui.Element {
+	k := pixel.K(c)
+	if len(levels) == 0 {
+		panic("media: Visualizer has no levels")
+	}
+	parts := make([]string, len(levels))
+	for i, l := range levels {
+		if !(l >= 0 && l <= 1) {
+			panic(fmt.Sprintf("media: Visualizer level %v outside 0–1", l))
+		}
+		parts[i] = fmt.Sprintf("%.2f", l)
+	}
+	h := pixel.Or("Visualizer Height", o.Height, 32)
+	mid := h / 2
+	return art(c, k, text, 4*len(levels)-1, h).Description(strings.Join(parts, " ")).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		for i, l := range levels {
+			n := int(math.Round(l * float64(mid)))
+			if n == 0 {
+				continue
+			}
+			// Magic at the root, Accent above, an AccentLit tip.
+			x, deep := 4*i, n/2
+			g.Rect(x, mid-deep, 3, 2*deep, pixel.Magic)
+			g.Rect(x, mid-n+1, 3, n-1-deep, pixel.Accent)
+			g.Rect(x, mid+deep, 3, n-1-deep, pixel.Accent)
+			g.Rect(x, mid-n, 3, 1, pixel.AccentLit)
+			g.Rect(x, mid+n-1, 3, 1, pixel.AccentLit)
+		}
+	})
+}