Add the step-by-step fixer and library types

Each pair is a Music (Artist/Album/01 Title) or Soundtracks (Album/01 Title)
library. The new Fixer tab takes the master, then the slave, through four
reviewed steps: folder structure, album tags (optional MusicBrainz lookup;
the slave reuses the fixed master's tags), folder names and file names,
then links the two libraries. Every fix is an undoable operation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/naming.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"path/filepath"
+	"reflect"
 	"regexp"
 	"strings"
 	"unicode"
@@ -79,6 +80,22 @@ func DefaultNaming() NamingConfig {
 	}
 }
 
+// Library types. Music is organized Artist/Album/01 Title; soundtracks (games,
+// films) Album/01 Title, since their "artist" is often a composer or empty.
+const (
+	LibraryMusic       = "music"
+	LibrarySoundtracks = "soundtracks"
+)
+
+// DefaultNamingFor returns the recommended naming for a library type.
+func DefaultNamingFor(libraryType string) NamingConfig {
+	c := DefaultNaming()
+	if libraryType == LibrarySoundtracks {
+		c.ArtistFolder = ""
+	}
+	return c
+}
+
 // SoundtrackFolder is the suggested folder rule for game and film soundtracks.
 var SoundtrackFolder = FolderNaming{Folder: "Video Games", AlbumFolder: "{Album Title}",
 	TrackFormat: "{track:00} {Track Title}", MultiDiscFormat: "{Medium Format} {medium:00}/{track:00} {Track Title}"}
@@ -103,7 +120,7 @@ var NamingPresets = []struct {
 // NamingOf returns a pair's naming, converting the settings of earlier
 // versions (one path pattern plus a disc style) when none is stored yet.
 func NamingOf(p store.Pair) NamingConfig {
-	c := DefaultNaming()
+	c := DefaultNamingFor(p.LibraryType)
 	if len(p.OrganizeConfig) > 0 && string(p.OrganizeConfig) != "{}" {
 		if stored, err := parseNaming(p.OrganizeConfig); err == nil {
 			return stored
@@ -139,7 +156,7 @@ func NamingOf(p store.Pair) NamingConfig {
 // parseNaming reads stored settings. Fields missing from the JSON take the
 // defaults; an artist folder that is present but empty stays empty.
 func parseNaming(raw []byte) (NamingConfig, error) {
-	c := DefaultNaming()
+	c := DefaultNaming() // missing keys: only very old or hand-made settings
 	if err := json.Unmarshal(raw, &c); err != nil {
 		return c, err
 	}
@@ -147,6 +164,17 @@ func parseNaming(raw []byte) (NamingConfig, error) {
 	return c, nil
 }
 
+// ChangeLibraryType switches a pair's naming to the new type's recommended
+// naming when it still uses the old type's, so custom naming is kept.
+func ChangeLibraryType(old store.Pair, p *store.Pair) {
+	if old.ID == 0 || old.LibraryType == p.LibraryType {
+		return
+	}
+	if reflect.DeepEqual(NamingOf(*p), DefaultNamingFor(old.LibraryType)) {
+		p.OrganizeConfig = nil
+	}
+}
+
 func fill(c *NamingConfig, d NamingConfig) {
 	c.ArtistFolder = strings.TrimSpace(c.ArtistFolder)
 	if c.Folders == nil {