Interface SyncDelta

The five fields receiveSyncChanges reads. Named as the native call names them, not as a /sync response names them: the two vocabularies have no member in common, and renaming here would only move the rename into a place with no compile-time help.

Every field is optional and defaults independently, native-side, when absent. Omit a field the caller has nothing for; do not set it to undefined explicitly instead -- encryptionSlice (in facade.ts) only ever assigns a key it has a real value for, and a hand-built SyncDelta should follow the same rule so Object.keys on the result means what it looks like it means.