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.

pixel/pad.go

@@ -9,6 +9,11 @@ func Pad(c *ui.Context, text string, cols, n, cw, ch int, cur *int, disabled boo
 		return ui.Box(c).Focusable().Role(ui.RoleGroup).Label(text).Disabled(disabled)
 	})
 	shell.Size(k.D(cols*cw), k.D((n+cols-1)/cols*ch))
+	picked = -1
+	// Disabled or inert, even a key already sent waits.
+	if base.IsDisabled() {
+		return shell, base, st, picked
+	}
 	for _, m := range []struct {
 		key ui.Key
 		to  func() int
@@ -21,7 +26,6 @@ func Pad(c *ui.Context, text string, cols, n, cw, ch int, cur *int, disabled boo
 			*cur = to
 		}
 	}
-	picked = -1
 	enter, space := base.Shortcut(0, ui.KeyEnter), base.Shortcut(0, ui.KeySpace)
 	if enter || space {
 		picked = *cur