perf(downloads): stream concurrent native queue work

lib/services/download_request_payload.dart

@@ -60,6 +60,7 @@ class DownloadRequestPayload {
   final String qualityVariant;
   final bool qualityVariantCollisionOnly;
   final String songLinkRegion;
+  final int networkConcurrencyLimit;
 
   const DownloadRequestPayload({
     this.contractVersion = nativeWorkerContractVersion,
@@ -121,6 +122,7 @@ class DownloadRequestPayload {
     this.qualityVariant = '',
     this.qualityVariantCollisionOnly = false,
     this.songLinkRegion = 'US',
+    this.networkConcurrencyLimit = 3,
   });
 
   Map<String, dynamic> toJson() {
@@ -184,6 +186,7 @@ class DownloadRequestPayload {
       'quality_variant': qualityVariant,
       'quality_variant_collision_only': qualityVariantCollisionOnly,
       'songlink_region': songLinkRegion,
+      'network_concurrency_limit': networkConcurrencyLimit,
     };
   }
 
@@ -251,6 +254,7 @@ class DownloadRequestPayload {
       qualityVariant: qualityVariant,
       qualityVariantCollisionOnly: qualityVariantCollisionOnly,
       songLinkRegion: songLinkRegion,
+      networkConcurrencyLimit: networkConcurrencyLimit,
     );
   }
 }

lib/services/platform_bridge.dart

@@ -1192,6 +1192,34 @@ class PlatformBridge {
     }
   }
 
+  static Future<void> appendNativeDownloadWorkerRequests({
+    required String runId,
+    required List<Map<String, dynamic>> requests,
+  }) async {
+    if (requests.isEmpty) return;
+    final payloadDir = await _nativeWorkerPayloadDir();
+    final stamp = DateTime.now().microsecondsSinceEpoch;
+    final requestPath = '${payloadDir.path}/append_$stamp.json';
+    await File(requestPath).writeAsString(jsonEncode(requests), flush: true);
+    try {
+      await _channel.invokeMethod('appendNativeDownloadWorkerRequests', {
+        'run_id': runId,
+        'requests_path': requestPath,
+      });
+    } catch (_) {
+      unawaited(_deleteFileIfExists(requestPath));
+      rethrow;
+    }
+  }
+
+  static Future<void> finishNativeDownloadWorkerPreparation({
+    required String runId,
+  }) async {
+    await _channel.invokeMethod('finishNativeDownloadWorkerPreparation', {
+      'run_id': runId,
+    });
+  }
+
   static Future<void> _deleteFileIfExists(String path) async {
     try {
       final file = File(path);