engine: make InstancePayload payload type explicit

This will be useful for deserialization use cases, e.g. for editing an instance-in-progress, where the payload may have been through a round trip to/from a persistence layer.

Author: eyelidlessness

packages/xforms-engine/src/client/serialization/InstancePayload.ts

@@ -79,6 +79,7 @@ type InstancePayloadViolation =
 	| MaxSizeViolation;
 
 interface BaseInstancePayload<PayloadType extends InstancePayloadType> {
+	readonly payloadType: PayloadType;
 	readonly status: InstancePayloadStatus;
 	readonly submissionMeta: SubmissionMeta;
 

packages/xforms-engine/src/lib/client-reactivity/instance-state/prepareInstancePayload.ts

@@ -103,6 +103,7 @@ const monolithicInstancePayload = (
 	const data = InstanceData.from(instanceFile, attachments);
 
 	return {
+		payloadType: 'monolithic',
 		...validation,
 		submissionMeta,
 		data: [data],
@@ -127,6 +128,7 @@ const chunkedInstancePayload = (
 	const data = InstanceData.from(instanceFile, attachments);
 
 	return {
+		payloadType: 'chunked',
 		...validation,
 		submissionMeta,
 		data: [data],