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/tracker.go

@@ -0,0 +1,119 @@
+package media
+
+import (
+	"fmt"
+	"log/slog"
+	"regexp"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+// Row is one tracker line; no Note is a rest.
+type Row struct {
+	// Note is like "C-4" or "C#4".
+	Note string
+	// Instrument is 1–99 and Volume 0–64 under a note.
+	Instrument, Volume int
+}
+
+type TrackerOptions struct {
+	Disabled bool
+}
+
+type TrackerResult struct {
+	Element *ui.Element
+	Changed bool
+}
+
+var noteForm = regexp.MustCompile(`^[A-G][-#][0-9]$`)
+
+// Tracker edits rows: piano letters write notes, Delete rests.
+func Tracker(c *ui.Context, text string, rows []Row, cur *int, o TrackerOptions) TrackerResult {
+	k := pixel.K(c)
+	if *cur < 0 || *cur >= len(rows) {
+		panic(fmt.Sprintf("media: Tracker cursor %d of %d rows", *cur, len(rows)))
+	}
+	for i, r := range rows {
+		if r.Note != "" && (!noteForm.MatchString(r.Note) || r.Instrument < 1 || r.Instrument > 99 || r.Volume < 0 || r.Volume > 64) {
+			panic(fmt.Sprintf("media: Tracker row %d is %+v", i, r))
+		}
+	}
+	shell, base, st, _ := pixel.Pad(c, text, 1, len(rows), 52, 7, cur, o.Disabled)
+	res := TrackerResult{Element: shell}
+	if !base.IsDisabled() {
+		for _, semi := range played(base) {
+			write(rows, *cur, cell(semi))
+			res.Changed = true
+			slog.Debug("8bit: tracker", "label", text, "row", *cur, "note", rows[*cur].Note)
+			*cur = min(len(rows)-1, *cur+1)
+		}
+		if base.Shortcut(0, ui.KeyDelete) || base.Shortcut(0, ui.KeyBackspace) {
+			rows[*cur], res.Changed = Row{}, true
+			slog.Debug("8bit: tracker", "label", text, "row", *cur, "note", "")
+		}
+	}
+	base.Description(line(*cur, rows[*cur]))
+	at := *cur
+	shell.Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		g.Rect(0, 0, g.W, g.H, pixel.Well)
+		for i, row := range rows {
+			y := i * 7
+			num, ink := pixel.Faint, pixel.Ink
+			switch {
+			case i == at:
+				g.Rect(0, y, g.W, 7, pixel.Accent)
+				num, ink = pixel.AccentInk, pixel.AccentInk
+			case i%4 == 0:
+				g.Rect(0, y, g.W, 7, pixel.Face)
+			}
+			g.Micro(2, y+1, fmt.Sprintf("%02d", i), num, 1)
+			if row.Note == "" {
+				g.Micro(14, y+1, "--- .. ..", num, 1)
+				continue
+			}
+			ins, vol := pixel.Info, pixel.Good
+			if i == at {
+				ins, vol = ink, ink
+			}
+			g.Micro(14, y+1, row.Note, ink, 1)
+			g.Micro(30, y+1, fmt.Sprintf("%02d", row.Instrument), ins, 1)
+			g.Micro(42, y+1, fmt.Sprintf("%02d", row.Volume), vol, 1)
+		}
+		g.Finish(st)
+	})
+	return res
+}
+
+// cell is a semitone in tracker form, like "C-4".
+func cell(semi int) string {
+	n := note(semi)
+	if len(n) == 2 {
+		return n[:1] + "-" + n[1:]
+	}
+	return n
+}
+
+// write notes row i; a rest borrows the voice above.
+func write(rows []Row, i int, n string) {
+	if rows[i].Note == "" {
+		rows[i] = Row{Instrument: 1, Volume: 64}
+		for j := i - 1; j >= 0; j-- {
+			if rows[j].Note != "" {
+				rows[i] = rows[j]
+				break
+			}
+		}
+	}
+	rows[i].Note = n
+}
+
+// line reads a row aloud: number, note, instrument, volume.
+func line(i int, r Row) string {
+	if r.Note == "" {
+		return fmt.Sprintf("%02d ---", i)
+	}
+	return fmt.Sprintf("%02d %s %02d %02d", i, r.Note, r.Instrument, r.Volume)
+}