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%.

docs/plans/2026-07-19-scanner-engine-fuzzy-diff.md

@@ -84,11 +84,11 @@
 - [x] run full test suite (`go test ./...`) and `go vet ./...` — must pass before final task
 
 ### Task 6: Verify acceptance criteria
-- [ ] verify `FindMissingReleases`/`ScanArtist`/`ScanAll` meet spec: normalization + 0.85 fuzzy threshold, ignore `IsIgnored`, per-artist scoping
+- [x] verify `FindMissingReleases`/`ScanArtist`/`ScanAll` meet spec: normalization + 0.85 fuzzy threshold, ignore `IsIgnored`, per-artist scoping (confirmed via tests in scanner_test.go / scan_test.go; grep of diff.go + scan.go shows normalizing via normalize.NormalizeString, IsIgnored skip, ArtistID grouping)
-- [ ] verify `config.Scanner.FuzzyThreshold` default 0.85 is used when threshold arg is zero (or document the chosen contract)
+- [x] verify `config.Scanner.FuzzyThreshold` default 0.85 is used when threshold arg is zero (resolveThreshold in scanner.go returns DefaultThreshold=0.85 on zero; TestScanArtist_ZeroThresholdUsesDefault asserts zero==explicit default)
-- [ ] run full test suite (unit) — all green
+- [x] run full test suite (unit) — all green (`go test ./...` passes)
-- [ ] run `go vet ./...` and `gofmt -l ./internal ./cmd` — zero issues
+- [x] run `go vet ./...` and `gofmt -l ./internal ./cmd` — zero issues (fixed two unformatted test files)
-- [ ] verify test coverage of `internal/scanner` and `internal/normalize` (target 80%+)
+- [x] verify test coverage of `internal/scanner` and `internal/normalize` (target 80%+) — scanner 90.3%, normalize 100.0%
 
 ### Task 7: Update documentation
 - [ ] update `README.md` to note the Scanner Engine is implemented (compute-only; notifier/web pending)

internal/musicbrainz/sync_test.go

@@ -888,4 +888,3 @@ func TestSyncArtistDiscography_ResyncWithNotifications(t *testing.T) {
 		t.Errorf("expected 0 notifications after resync, got %d", count)
 	}
 }
-

internal/scanner/scan_test.go

@@ -82,6 +82,30 @@ func TestScanArtist(t *testing.T) {
 	}
 }
 
+func TestScanArtist_ZeroThresholdUsesDefault(t *testing.T) {
+	db := newTestDB(t)
+	defer db.Close()
+
+	seedArtist(t, db, "artist-1", "Pink Floyd")
+	seedLocalAlbum(t, db, "l1", "artist-1", "The Wall")
+	seedExternalRelease(t, db, "rg1", "artist-1", "The Wall", false)
+	seedExternalRelease(t, db, "rg2", "artist-1", "Animals", false)
+
+	// Pass 0 (zero value / unset) and the explicit default; results must match.
+	zero, err := ScanArtist(context.Background(), db, "artist-1", 0)
+	if err != nil {
+		t.Fatalf("ScanArtist(0) error: %v", err)
+	}
+	explicit, err := ScanArtist(context.Background(), db, "artist-1", DefaultThreshold)
+	if err != nil {
+		t.Fatalf("ScanArtist(%v) error: %v", DefaultThreshold, err)
+	}
+	if len(zero) != len(explicit) {
+		t.Errorf("ScanArtist(0) returned %d missing, ScanArtist(%v) returned %d; must match",
+			len(zero), DefaultThreshold, len(explicit))
+	}
+}
+
 func TestScanArtist_IgnoredNotReported(t *testing.T) {
 	db := newTestDB(t)
 	defer db.Close()