Implement incremental sync and offline recovery

packages/apple/VelodySync/Tests/VelodySyncTests/OfflineLibraryServiceTests.swift

@@ -176,7 +176,8 @@ final class OfflineLibraryServiceTests: XCTestCase {
         let syncService = RemoteLibrarySyncService(
             repository: DefaultRemoteLibraryRepository(
                 apiClient: OfflineLibraryMockAPIClient(audioAssetData: sampleMp3Data(seed: track.assetId)),
-                store: remoteLibraryStore
+                store: remoteLibraryStore,
+                syncCursorStore: InMemoryRemoteLibrarySyncCursorStore()
             ),
             downloadStateStore: downloadStateStore,
             audioFileStore: audioFileStore,
@@ -226,8 +227,10 @@ final class OfflineLibraryServiceTests: XCTestCase {
         let remoteLibraryStore = InMemoryRemoteLibraryStore()
         let audioData = sampleMp3Data(seed: track.assetId)
         let apiClient = OfflineLibraryMockAPIClient(
-            remoteLibraryResponse: RemoteLibraryResponseDTO(
-                tracks: [makeRemoteTrackDTO(from: track)]
+            bootstrapResponse: SyncBootstrapResponse(
+                nextCursor: SyncCursor(value: "1"),
+                tracks: [track],
+                serverTime: "2026-05-30T08:00:00.000Z"
             ),
             audioAssetData: audioData,
             artworkDataByArtworkID: [
@@ -240,7 +243,8 @@ final class OfflineLibraryServiceTests: XCTestCase {
         let syncService = RemoteLibrarySyncService(
             repository: DefaultRemoteLibraryRepository(
                 apiClient: apiClient,
-                store: remoteLibraryStore
+                store: remoteLibraryStore,
+                syncCursorStore: InMemoryRemoteLibrarySyncCursorStore()
             ),
             downloadStateStore: downloadStateStore,
             audioFileStore: audioFileStore,
@@ -284,8 +288,10 @@ final class OfflineLibraryServiceTests: XCTestCase {
             makeRemoteTrack(trackId: "track-2", assetId: "asset-2", title: "Track 2", artworkId: "artwork-2"),
         ]
         let apiClient = OfflineLibraryMockAPIClient(
-            remoteLibraryResponse: RemoteLibraryResponseDTO(
-                tracks: tracks.map { makeRemoteTrackDTO(from: $0) }
+            bootstrapResponse: SyncBootstrapResponse(
+                nextCursor: SyncCursor(value: "1"),
+                tracks: tracks,
+                serverTime: "2026-05-30T08:00:00.000Z"
             ),
             audioAssetDataByAssetID: [
                 "asset-1": sampleMp3Data(seed: "asset-1"),
@@ -304,7 +310,8 @@ final class OfflineLibraryServiceTests: XCTestCase {
 
         let firstRepository = DefaultRemoteLibraryRepository(
             apiClient: apiClient,
-            store: try FileRemoteLibraryStore(fileURL: remoteLibraryFileURL)
+            store: try FileRemoteLibraryStore(fileURL: remoteLibraryFileURL),
+            syncCursorStore: InMemoryRemoteLibrarySyncCursorStore()
         )
         let firstDownloadStateStore = try FileRemoteTrackDownloadStateStore(fileURL: downloadStateFileURL)
         let firstAudioStore = try FileOfflineAudioFileStore(baseDirectoryURL: audioDirectory)
@@ -329,7 +336,8 @@ final class OfflineLibraryServiceTests: XCTestCase {
 
         let relaunchedRepository = DefaultRemoteLibraryRepository(
             apiClient: apiClient,
-            store: try FileRemoteLibraryStore(fileURL: remoteLibraryFileURL)
+            store: try FileRemoteLibraryStore(fileURL: remoteLibraryFileURL),
+            syncCursorStore: InMemoryRemoteLibrarySyncCursorStore(cursor: SyncCursor(value: "1"))
         )
         let relaunchedDownloadStateStore = try FileRemoteTrackDownloadStateStore(fileURL: downloadStateFileURL)
         let relaunchedAudioStore = try FileOfflineAudioFileStore(baseDirectoryURL: audioDirectory)
@@ -404,18 +412,22 @@ private actor InMemoryRemoteLibraryRepository: RemoteLibraryRepository {
 }
 
 private struct OfflineLibraryMockAPIClient: VelodyAPIClient {
-    let remoteLibraryResponse: RemoteLibraryResponseDTO?
+    let bootstrapResponse: SyncBootstrapResponse
     let audioAssetData: Data?
     let audioAssetDataByAssetID: [String: Data]
     let artworkDataByArtworkID: [String: Data]
 
     init(
-        remoteLibraryResponse: RemoteLibraryResponseDTO? = nil,
+        bootstrapResponse: SyncBootstrapResponse = SyncBootstrapResponse(
+            nextCursor: SyncCursor(value: "0"),
+            tracks: [],
+            serverTime: "2026-05-30T08:00:00.000Z"
+        ),
         audioAssetData: Data? = nil,
         audioAssetDataByAssetID: [String: Data] = [:],
         artworkDataByArtworkID: [String: Data] = [:]
     ) {
-        self.remoteLibraryResponse = remoteLibraryResponse
+        self.bootstrapResponse = bootstrapResponse
         self.audioAssetData = audioAssetData
         self.audioAssetDataByAssetID = audioAssetDataByAssetID
         self.artworkDataByArtworkID = artworkDataByArtworkID
@@ -444,11 +456,17 @@ private struct OfflineLibraryMockAPIClient: VelodyAPIClient {
     }
 
     func fetchSyncBootstrap() async throws -> SyncBootstrapResponse {
-        SyncBootstrapResponse(
-            nextCursor: SyncCursor(value: "0"),
-            tracks: [],
+        bootstrapResponse
+    }
+
+    func fetchSyncChanges(
+        cursor: SyncCursor
+    ) async throws -> SyncChangesResponse {
+        SyncChangesResponse(
+            nextCursor: cursor,
+            hasMore: false,
+            requiresBootstrap: false,
             events: [],
-            deletedTrackIds: [],
             serverTime: "2026-05-30T08:00:00.000Z"
         )
     }
@@ -457,7 +475,9 @@ private struct OfflineLibraryMockAPIClient: VelodyAPIClient {
         deviceId: String
     ) async throws -> RemoteLibraryResponseDTO {
         _ = deviceId
-        return remoteLibraryResponse ?? RemoteLibraryResponseDTO(tracks: [])
+        return RemoteLibraryResponseDTO(
+            tracks: bootstrapResponse.tracks.map { makeRemoteTrackDTO(from: $0) }
+        )
     }
 
     func downloadAudioAsset(