New core/metadata/types.py with canonical dataclasses + classmethod
converters for spotify/itunes/deezer/discogs/musicbrainz/hydrabase.
Each converter is the single place that knows that provider's wire
shape — addresses the duck-typing pattern Cin flagged.
Pure additive: no consumer code changed. Follow-up PRs migrate
consumers one at a time. Migration plan at
docs/metadata-types-migration.md.
Tests: 32 cases pin per-provider semantics + cross-provider
invariants. Also stabilized a flaky discogs test that depended on
local config state.
