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%.
Add FindMissingReleases and MissingRelease to internal/scanner: skips IsIgnored external releases, scopes matches per ArtistID, and reports external releases with no local album above the fuzzy threshold.
