Mirror the library_schema and library_row_mapper split: HistorySchema owns
create, upgrade, index creation and backfills, HistoryRowMapper owns the
JSON/row mapping and the derived normalized and queue-sort columns. SQL text
is unchanged. Tests that read the history CREATE TABLE and index text now
read history_schema.dart.
Keep format choices available independently of loaded pages, match codec and extension aliases, and add a filter for indexed ReplayGain tags. Preserve source, quality, search, paging, and existing missing-ReplayGain behavior.
Validation: 13 targeted database and widget tests; Flutter localization generation and targeted analysis.
zarzet