CloudKitRecordMapper was a standalone actor, so 'await recordMapper.toCKRecord'
from the @MainActor sync engine hopped off the main actor and read live
SwiftData @Model properties (e.g. LocalPlaylistItem.authorName) from the wrong
executor. SwiftData models are bound to the main ModelContext and are not
thread-safe, so this raced the backing store and crashed with EXC_BAD_ACCESS.
Make the mapper @MainActor (it must touch main-isolated models regardless) and
drop the now-redundant awaits. Fixes the crash for all synced model types.
Fixes an EXC_BAD_ACCESS seen in TestFlight build 261.
ba33823048