Flag slave-only songs whose album or artist the master has

Scans now mark slave-only songs as missing from the master when the master
has their album folder or at least their artist folder. A new Missing from
master filter in Review lists them, split by album and artist.

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

internal/engine/scan.go

@@ -105,6 +105,8 @@ type masterIndex struct {
 	byDir   map[string][]string // lower(dir) -> rels
 	byTitle map[string][]string // lower(top folder)|normTitle -> rels
 
+	folders masterFolders // artist and album folders, for songs missing from the master
+
 	approx    *approxIndex // nil unless the pair enables approximate matching
 	threshold float64
 	verify    VerifyFunc // confirms library-wide approximate matches; nil disables that scope
@@ -113,6 +115,7 @@ type masterIndex struct {
 func buildIndex(files map[string]fsops.Info, p store.Pair) *masterIndex {
 	ix := &masterIndex{files: files, lower: map[string]string{}, byDir: map[string][]string{}, byTitle: map[string][]string{}}
 	rels := sortedKeys(files)
+	ix.folders = buildMasterFolders(rels)
 	if p.ApproxMatching {
 		ix.approx = buildApproxIndex(rels)
 		ix.threshold = float64(p.ApproxThreshold) / 100
@@ -327,6 +330,9 @@ func (e *Engine) scan(ctx context.Context, j *Job, pairID int64, trigger string)
 			pm = &p
 		}
 		e.classify(ctx, l, &m, masterFiles[m.RelMaster], si, pm)
+		if m.Status == store.StatusUnmatched {
+			m.Missing, m.Reason = ix.folders.missing(rel)
+		}
 		if err := e.st.UpsertMatch(ctx, &m); err != nil {
 			return err
 		}
@@ -483,6 +489,9 @@ func (e *Engine) refresh(ctx context.Context, l *libs, relSlave string, hint ...
 		m.Kind = store.KindNone
 	}
 	e.classify(ctx, l, &m, mi, si, pm)
+	if m.Status == store.StatusUnmatched && pm != nil && pm.Status == store.StatusUnmatched {
+		m.Missing, m.Reason = pm.Missing, pm.Reason // the next scan re-checks the master's folders
+	}
 	_ = e.st.UpsertMatch(ctx, &m)
 }