Stop warning about slave-only content in layout checks

A singles library naturally has artists and songs the albums library
lacks, so "only N of M top-level folders exist in the master" and "N of M
slave files have no counterpart" were false alarms.

Layout warnings now only flag real layout problems:
- Folders that are the same but named differently (case, punctuation,
  accents, "&"/"and", leading/trailing "The"), at the top level and one
  level down inside shared folders, with examples.
- Songs found in both libraries that mostly live in a different folder in
  the master (e.g. Artist/Song vs Artist/Album/Song). Slave-only songs are
  ignored, as are singles/EPs/compilations matched to their album version.

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

internal/engine/engine.go

@@ -298,43 +298,26 @@ func (e *Engine) NormalizePair(p *store.Pair) ([]string, error) {
 	return layoutWarnings(l), nil
 }
 
-// layoutWarnings does a cheap top-level comparison of the two libraries.
-// Any folder layout works; what matters is that both libraries use the same
-// one, so matching files sit at the same relative path.
+// layoutWarnings is the quick check run when a pair is saved and at the
+// start of each scan. Content that exists in only one library is normal and
+// never warned about; only folders that are the same but named differently are.
 func layoutWarnings(l *libs) []string {
-	var warns []string
-	list := func(root string) (dirs map[string]bool, files int) {
-		dirs = map[string]bool{}
-		ents, _ := os.ReadDir(root)
+	empty := true
+	if ents, _ := os.ReadDir(l.slave.Path); len(ents) > 0 {
 		for _, en := range ents {
-			if strings.HasPrefix(en.Name(), ".") {
-				continue
-			}
-			if en.IsDir() {
-				dirs[strings.ToLower(en.Name())] = true
-			} else if l.hasExt(en.Name()) {
-				files++
+			if !strings.HasPrefix(en.Name(), ".") && (en.IsDir() || l.hasExt(en.Name())) {
+				empty = false
+				break
 			}
 		}
-		return
 	}
-	md, _ := list(l.master.Path)
-	sd, sf := list(l.slave.Path)
-	if len(sd) == 0 && sf == 0 {
-		warns = append(warns, "The slave library is empty. Use Build to pick songs from the master library to hardlink into it.")
-		return warns
+	if empty {
+		return []string{"The slave library is empty. Use Build to pick songs from the master library to hardlink into it."}
 	}
-	shared := 0
-	for d := range sd {
-		if md[d] {
-			shared++
-		}
+	if w := renamedWarning(renamedFolders(l)); w != "" {
+		return []string{w}
 	}
-	if len(sd) > 0 && shared*2 < len(sd) {
-		warns = append(warns, fmt.Sprintf("Only %d of %d top-level folders in the slave library also exist in the master library. "+
-			"Both libraries should use the same folder layout, or most files will not match.", shared, len(sd)))
-	}
-	return warns
+	return nil
 }
 
 // ---- helpers ----