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/notes.go

@@ -0,0 +1,34 @@
+package media
+
+import (
+	"fmt"
+
+	"github.com/egoist/mygo/ui"
+)
+
+// keyboard plays C4–C5: A–K white, WETYU black.
+var keyboard = [13]struct {
+	key    ui.Key
+	letter string
+}{
+	{ui.KeyA, "A"}, {ui.KeyW, "W"}, {ui.KeyS, "S"}, {ui.KeyE, "E"}, {ui.KeyD, "D"}, {ui.KeyF, "F"}, {ui.KeyT, "T"},
+	{ui.KeyG, "G"}, {ui.KeyY, "Y"}, {ui.KeyH, "H"}, {ui.KeyU, "U"}, {ui.KeyJ, "J"}, {ui.KeyK, "K"},
+}
+
+var names = [12]string{"C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"}
+
+// note names the semitone above C4, like "C#4".
+func note(semi int) string {
+	return fmt.Sprintf("%s%d", names[semi%12], 4+semi/12)
+}
+
+// played are the semitones whose letters e heard this frame.
+func played(e *ui.Element) []int {
+	var out []int
+	for semi, kb := range keyboard {
+		if e.Shortcut(0, kb.key) {
+			out = append(out, semi)
+		}
+	}
+	return out
+}