feat: verify scanner engine acceptance criteria

Confirm normalization + 0.85 threshold, IsIgnored skip, per-artist scoping,
and zero-threshold→default contract. Added TestScanArtist_ZeroThresholdUsesDefault,
fixed gofmt on two test files. Coverage: scanner 90.3%, normalize 100.0%.

internal/scanner/scanner_test.go

@@ -8,11 +8,11 @@ import (
 
 func TestSimilarity(t *testing.T) {
 	tests := []struct {
-		name     string
-		a        string
-		b        string
-		want     float64
-		epsilon  float64
+		name    string
+		a       string
+		b       string
+		want    float64
+		epsilon float64
 	}{
 		{
 			name:    "exact match scores 1.0",
@@ -127,14 +127,14 @@ func TestFindMissingReleases(t *testing.T) {
 	artistB := "artist-b"
 
 	tests := []struct {
-		name    string
-		local   []database.LocalAlbum
+		name     string
+		local    []database.LocalAlbum
 		external []database.ExternalRelease
-		want    []string // RGIDs expected to be reported as missing
+		want     []string // RGIDs expected to be reported as missing
 	}{
 		{
-			name:     "no local albums means all external are missing",
-			local:    nil,
+			name:  "no local albums means all external are missing",
+			local: nil,
 			external: []database.ExternalRelease{
 				{RGID: "rg1", ArtistID: artistA, Title: "The Wall"},
 				{RGID: "rg2", ArtistID: artistA, Title: "Animals"},