fix: address code review findings

internal/scanner/scanner.go

@@ -59,7 +59,10 @@ func Similarity(a, b string) float64 {
 }
 
 // IsMatch reports whether a and b are similar enough to be considered the
-// same release, given the provided threshold in [0.0, 1.0].
+// same release, given the provided threshold in [0.0, 1.0]. A threshold of 0
+// (unset) falls back to DefaultThreshold, so this primitive honors the same
+// zero-means-default contract as FindMissingReleases/ScanArtist/ScanAll rather
+// than treating 0 as "always match".
 func IsMatch(a, b string, threshold float64) bool {
-	return Similarity(a, b) >= threshold
+	return Similarity(a, b) >= resolveThreshold(threshold)
 }