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/scan.go

@@ -314,7 +314,7 @@ func (e *Engine) scan(ctx context.Context, j *Job, pairID int64, trigger string)
 
 	j.phase("Comparing files", int64(len(slaveFiles)))
 	keep := map[string]bool{}
-	kinds := map[string]int{}
+	var layout layoutStats
 	for _, rel := range sortedKeys(slaveFiles) {
 		if ctx.Err() != nil {
 			return ctx.Err()
@@ -331,7 +331,7 @@ func (e *Engine) scan(ctx context.Context, j *Job, pairID int64, trigger string)
 			return err
 		}
 		keep[rel] = true
-		kinds[m.Kind]++
+		layout.add(m.RelSlave, m.RelMaster, m.Kind, m.Reason)
 		j.count(m.Status)
 		j.tick()
 	}
@@ -340,11 +340,8 @@ func (e *Engine) scan(ctx context.Context, j *Job, pairID int64, trigger string)
 	}
 
 	// Layout warnings based on what was actually found.
-	if ss.files > 0 {
-		if n := kinds[store.KindElsewhere] + kinds[store.KindApprox] + kinds[store.KindNone]; n*4 > ss.files {
-			warns = append(warns, fmt.Sprintf("%d of %d slave files have no counterpart at the same path in the master library. "+
-				"Check that both libraries use the same folder layout.", n, ss.files))
-		}
+	if w := layout.warning(); w != "" {
+		warns = append(warns, w)
 	}
 	if ms.symlinks+ss.symlinks > 0 {
 		warns = append(warns, fmt.Sprintf("Ignored %d symlinks (playlink never follows or replaces symlinks).", ms.symlinks+ss.symlinks))