perf: optimize native worker snapshot writes with delta mode

Replace full-queue snapshot writes on every progress tick with a
lightweight delta mode that only includes the active item.

- Progress tick (1s loop) now writes item_delta with only the
  active item instead of serializing the entire queue
- Full compact_items snapshots are reserved for important events:
  start, pause/resume/cancel, item boundaries, finish, and
  service stop/destroy
- Compact items omit large static fields (item_json, track_name,
  artist_name) that Dart already has from queue restore
- Snapshot always carries item_ids for adoption correlation
- Dart-side _applyAndroidNativeWorkerSnapshot handles item_delta
  as a single-item fallback when items array is absent
- Dart-side _tryAdoptAndroidNativeWorkerSnapshot reads item_ids
  as fallback when items is not present
- Add deferred SAF publish: native worker writes to cache, runs
  all finalization locally, then publishes once to SAF at the end
- Forward defer_saf_publish through DownloadRequestPayload

lib/providers/download_queue_provider.dart

@@ -4520,15 +4520,22 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
     }
 
     final rawItems = snapshot['items'];
-    if (rawItems is! List || rawItems.isEmpty) {
+    final rawItemIds = snapshot['item_ids'];
+    final snapshotIds = rawItems is List
+        ? rawItems
+              .whereType<Map<Object?, Object?>>()
+              .map((item) => item['item_id']?.toString() ?? '')
+              .where((id) => id.isNotEmpty)
+              .toSet()
+        : rawItemIds is List
+        ? rawItemIds
+              .map((id) => id?.toString() ?? '')
+              .where((id) => id.isNotEmpty)
+              .toSet()
+        : <String>{};
+    if (snapshotIds.isEmpty) {
       return false;
     }
-
-    final snapshotIds = rawItems
-        .whereType<Map<Object?, Object?>>()
-        .map((item) => item['item_id']?.toString() ?? '')
-        .where((id) => id.isNotEmpty)
-        .toSet();
     if (!restoredItems.any((item) => snapshotIds.contains(item.id))) {
       return false;
     }
@@ -4958,6 +4965,7 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
       safFileName: safFileName ?? '',
       safOutputExt: safOutputExt,
       stageSafOutput: isSafMode,
+      deferSafPublish: isSafMode,
       requiresContainerConversion:
           outputExt == '.flac' &&
           _extensionRequiresNativeContainerConversion(item.service),
@@ -4984,13 +4992,23 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
     AppSettings settings,
   ) async {
     final rawItems = snapshot['items'];
-    if (rawItems is! List) {
+    final rawDelta = snapshot['item_delta'];
+    final itemSnapshots = <Map<String, dynamic>>[];
+    if (rawItems is List) {
+      for (final rawItem in rawItems) {
+        if (rawItem is Map) {
+          itemSnapshots.add(Map<String, dynamic>.from(rawItem));
+        }
+      }
+    }
+    if (rawDelta is Map) {
+      itemSnapshots.add(Map<String, dynamic>.from(rawDelta));
+    }
+    if (itemSnapshots.isEmpty) {
       return;
     }
 
-    for (final rawItem in rawItems) {
-      if (rawItem is! Map) continue;
-      final itemSnapshot = Map<String, dynamic>.from(rawItem);
+    for (final itemSnapshot in itemSnapshots) {
       final itemId = itemSnapshot['item_id']?.toString() ?? '';
       if (itemId.isEmpty || reconciledIds.contains(itemId)) {
         continue;

lib/services/download_request_payload.dart

@@ -44,6 +44,7 @@ class DownloadRequestPayload {
   final String safFileName;
   final String safOutputExt;
   final bool stageSafOutput;
+  final bool deferSafPublish;
   final bool requiresContainerConversion;
   final String songLinkRegion;
 
@@ -91,6 +92,7 @@ class DownloadRequestPayload {
     this.safFileName = '',
     this.safOutputExt = '',
     this.stageSafOutput = false,
+    this.deferSafPublish = false,
     this.requiresContainerConversion = false,
     this.songLinkRegion = 'US',
   });
@@ -140,6 +142,7 @@ class DownloadRequestPayload {
       'saf_file_name': safFileName,
       'saf_output_ext': safOutputExt,
       'stage_saf_output': stageSafOutput,
+      'defer_saf_publish': deferSafPublish,
       'requires_container_conversion': requiresContainerConversion,
       'songlink_region': songLinkRegion,
     };
@@ -193,6 +196,7 @@ class DownloadRequestPayload {
       safFileName: safFileName,
       safOutputExt: safOutputExt,
       stageSafOutput: stageSafOutput,
+      deferSafPublish: deferSafPublish,
       requiresContainerConversion: requiresContainerConversion,
       songLinkRegion: songLinkRegion,
     );