Move worker request contract validation and the download history row
and JSON mapping out of NativeDownloadFinalizer into a sibling file of
extension functions. Bodies are verbatim. NATIVE_WORKER_CONTRACT_VERSION
stays on the object with its value, so its contract test is unchanged.
The history column contract test now reads buildHistoryRow from the new
file. normalizeOptional moves with the row builder, so
cleanMetadataString widens from private to internal.
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.
The grab-bag file mixed model tests with tests that pin literal source
text across Dart, Kotlin and Swift. The contracts now live in
source_contracts_test.dart with a header explaining that they must be
updated, never deleted, when code moves. The remaining groups moved
verbatim into domain files.
zarzet