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/waveform.go

@@ -0,0 +1,40 @@
+package media
+
+import (
+	"fmt"
+	"math"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type WaveformOptions struct {
+	// Height is in dots; zero means 32.
+	Height int
+}
+
+// Waveform draws samples, ±1, as lines from the middle.
+func Waveform(c *ui.Context, text string, samples []float64, o WaveformOptions) *ui.Element {
+	k := pixel.K(c)
+	if len(samples) == 0 {
+		panic("media: Waveform has no samples")
+	}
+	for _, s := range samples {
+		if !(s >= -1 && s <= 1) {
+			panic(fmt.Sprintf("media: Waveform sample %v outside ±1", s))
+		}
+	}
+	h := pixel.Or("Waveform Height", o.Height, 32)
+	half := (h - 1) / 2
+	return art(c, k, text, 2*len(samples)-1, h).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		for x := 1; x < g.W; x += 2 {
+			g.Dot(x, half, pixel.Faint)
+		}
+		for i, s := range samples {
+			n := int(math.Round(s * float64(half)))
+			g.VLine(2*i, min(half, half-n), max(n, -n)+1, pixel.Accent)
+		}
+	})
+}