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/layout_test.go

@@ -0,0 +1,104 @@
+package engine
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func scanWarnings(t *testing.T, ev *env) []string {
+	t.Helper()
+	ev.scan()
+	p, err := ev.st.Pair(ctx, ev.pair.ID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var out []string
+	for _, w := range p.LastScanWarnings {
+		if !strings.Contains(w, "ffprobe") {
+			out = append(out, w)
+		}
+	}
+	return out
+}
+
+// The user's situation: a singles library with far more artists and songs
+// than the albums library. Slave-only content must not trigger warnings.
+func TestSlaveOnlyContentIsNotALayoutProblem(t *testing.T) {
+	ev := setup(t, false)
+	for i := 0; i < 40; i++ {
+		ev.write(ev.slave, fmt.Sprintf("Singles Artist %02d/Some Single/Song.mp3", i), "s")
+	}
+	for i := 0; i < 10; i++ {
+		rel := fmt.Sprintf("Shared Artist/Album/%02d Track.flac", i)
+		ev.write(ev.master, rel, "same")
+		ev.write(ev.slave, rel, "same")
+	}
+	ev.write(ev.master, "Master Only/Album/01 X.flac", "m")
+	if w := scanWarnings(t, ev); len(w) != 0 {
+		t.Fatalf("slave-only content should not produce warnings, got %q", w)
+	}
+}
+
+func TestRenamedFoldersAreReported(t *testing.T) {
+	ev := setup(t, false)
+	ev.write(ev.master, "AC_DC/Back in Black/01 Hells Bells.flac", "a")
+	ev.write(ev.slave, "AC-DC/Back in Black/01 Hells Bells.mp3", "b")
+	ev.write(ev.master, "The Beatles/Abbey Road/01 Come Together.flac", "a")
+	ev.write(ev.slave, "Beatles, The/Abbey Road/01 Come Together.mp3", "b")
+	ev.write(ev.master, "Avenged Sevenfold/City of Evil/03 Bat Country.flac", "a")
+	ev.write(ev.slave, "Avenged Sevenfold/City Of Evil/03 Bat Country.mp3", "b")
+	ev.write(ev.slave, "Beyoncé/Lemonade/Formation.mp3", "b")
+	ev.write(ev.master, "Beyonce/Lemonade/Formation.flac", "a")
+	w := scanWarnings(t, ev)
+	if len(w) != 1 {
+		t.Fatalf("want one renamed-folders warning, got %q", w)
+	}
+	for _, want := range []string{`"AC-DC" (slave) vs "AC_DC" (master)`, `"Avenged Sevenfold/City Of Evil" (slave) vs "Avenged Sevenfold/City of Evil" (master)`, "4 folders", "1 more"} {
+		if !strings.Contains(w[0], want) {
+			t.Errorf("warning %q should contain %q", w[0], want)
+		}
+	}
+}
+
+// A slave library without album folders is a genuine layout difference.
+func TestDifferentLayoutIsReported(t *testing.T) {
+	ev := setup(t, false)
+	for i := 0; i < 24; i++ {
+		ev.write(ev.master, fmt.Sprintf("Band/Album/%02d Song Number %d.flac", i+1, i), "m")
+		ev.write(ev.slave, fmt.Sprintf("Band/Song Number %d.mp3", i), "s")
+	}
+	w := scanWarnings(t, ev)
+	if len(w) != 1 || !strings.Contains(w[0], "Of 24 songs found in both libraries, 24 are in a different folder") {
+		t.Fatalf("want a different-layout warning, got %q", w)
+	}
+}
+
+// Singles matched to their album version are intended, not a layout problem.
+func TestSinglesMatchedToAlbumsAreNotALayoutProblem(t *testing.T) {
+	ev := setup(t, false)
+	enableApprox(t, ev)
+	for i := 0; i < 24; i++ {
+		ev.write(ev.master, fmt.Sprintf("Band/Album/%02d Song Number %d.flac", i+1, i), "m")
+		ev.write(ev.slave, fmt.Sprintf("Band/Song Number %d - Single/Song Number %d.mp3", i, i), "s")
+	}
+	if w := scanWarnings(t, ev); len(w) != 0 {
+		t.Fatalf("singles matched to album versions should not warn, got %q", w)
+	}
+}
+
+func TestFolderKey(t *testing.T) {
+	same := [][2]string{{"AC/DC", "AC-DC"}, {"The Beatles", "Beatles, The"}, {"Simon & Garfunkel", "Simon and Garfunkel"},
+		{"Beyoncé", "Beyonce"}, {"City Of Evil", "City of Evil"}, {"Sigur Rós", "Sigur Ros"}}
+	for _, p := range same {
+		if folderKey(p[0]) != folderKey(p[1]) {
+			t.Errorf("%q and %q should share a key (%q vs %q)", p[0], p[1], folderKey(p[0]), folderKey(p[1]))
+		}
+	}
+	if folderKey("Theory of a Deadman") == folderKey("ory of a Deadman") {
+		t.Error(`"The" must only be stripped as a whole word`)
+	}
+	if folderKey("Nightmare") == folderKey("Nightmare (Deluxe)") {
+		t.Error("different folders must keep different keys")
+	}
+}