feat: Sample Tag Viewer — read-only tag browser for AKAI disk images

Backend:
- extract.go: listTags() navigates to volume, runs akaiutil lstags
- extract.go: parseTags() parses tag name/index from lstags output
- serve.go: GET /api/disk/partitions — list partitions on an ISO
- serve.go: GET /api/disk/volumes — list volumes on a partition
- serve.go: GET /api/volume/tags — list tags on a volume

Web UI:
- Tag Viewer section with ISO path input and Browse button
- Partition selector buttons
- Volume grid with click-to-select
- Tag chips with color-coded dots (20-color palette)

Tests:
- 6 parseTags tests (basic, spaces, defaults, no tags, empty, no-type)
- 1 listTags integration test
- 4 API endpoint tests (partitions, missing-ISO, missing-params, tags)

Karpathy: read-only view first — no write operations yet (settagi, clrtagi)

extract.go

@@ -71,8 +71,8 @@ var runAkaiutil = func(isoPath, commands string) (string, error) {
 }
 
 type volInfo struct {
-	Part string
-	Name string
+	Part string `json:"part"`
+	Name string `json:"name"`
 }
 
 func listPartitions(isoPath string) ([]string, error) {
@@ -210,3 +210,54 @@ var extractISO = func(isoPath, outDir string) (string, error) {
 	}
 	return fmt.Sprintf("Total WAV files: %d (%s)", total, elapsed), nil
 }
+
+type TagInfo struct {
+	Index int    `json:"index"`
+	Name  string `json:"name"`
+}
+
+func listTags(isoPath, part, volName string) ([]TagInfo, error) {
+	var cmds strings.Builder
+	fmt.Fprintf(&cmds, "df\n")
+	fmt.Fprintf(&cmds, "cd /disk0/%s\n", part)
+	nav := strings.ReplaceAll(volName, " ", "_")
+	fmt.Fprintf(&cmds, "cd /disk0/%s/%s\n", part, nav)
+	fmt.Fprintf(&cmds, "lstags\n")
+	cmds.WriteString("q\n")
+
+	out, err := runAkaiutil(isoPath, cmds.String())
+	if err != nil {
+		return nil, fmt.Errorf("list tags: %w", err)
+	}
+
+	return parseTags(out), nil
+}
+
+func parseTags(output string) []TagInfo {
+	reTag := regexp.MustCompile(`^\s*(\d+)\s+(\S.*)`)
+
+	var tags []TagInfo
+	inTags := false
+
+	for _, line := range strings.Split(output, "\n") {
+		trimmed := strings.TrimRight(line, " \r")
+		if strings.Contains(trimmed, "tag  name") {
+			inTags = true
+			continue
+		}
+		if !inTags {
+			continue
+		}
+		if strings.HasPrefix(trimmed, "---") && len(tags) > 0 {
+			break
+		}
+		if m := reTag.FindStringSubmatch(trimmed); m != nil {
+			idx, _ := strconv.Atoi(m[1])
+			name := strings.TrimSpace(m[2])
+			if name != "" {
+				tags = append(tags, TagInfo{Index: idx, Name: name})
+			}
+		}
+	}
+	return tags
+}