perf: incremental download queue lookup updates, async cover cleanup, and native JSON decoding on iOS

- Embed DownloadQueueLookup into DownloadQueueState; add updatedForIndices() for O(changed) incremental updates during frequent progress ticks instead of full O(n) rebuild
- downloadQueueLookupProvider now reads pre-computed lookup from state directly
- Replace sync file deletion in DownloadedEmbeddedCoverResolver with unawaited async cleanup to avoid blocking the main thread
- Parse JSON payloads on iOS native side (parseJsonPayload) so event sinks and method channel responses return native objects, avoiding redundant Dart-side JSON decode
- Use .cast<String, dynamic>() instead of Map.from() in _decodeMapResult for zero-copy map handling

ios/Runner/AppDelegate.swift

@@ -135,7 +135,7 @@ import Gobackend  // Import Go framework
             }
             self.lastDownloadProgressPayload = payload
             DispatchQueue.main.async { [weak self] in
-                self?.downloadProgressEventSink?(payload)
+                self?.downloadProgressEventSink?(self?.parseJsonPayload(payload))
             }
         }
         downloadProgressTimer = timer
@@ -165,7 +165,7 @@ import Gobackend  // Import Go framework
             }
             self.lastLibraryScanProgressPayload = payload
             DispatchQueue.main.async { [weak self] in
-                self?.libraryScanProgressEventSink?(payload)
+                self?.libraryScanProgressEventSink?(self?.parseJsonPayload(payload))
             }
         }
         libraryScanProgressTimer = timer
@@ -179,6 +179,17 @@ import Gobackend  // Import Go framework
         libraryScanProgressEventSink = nil
         lastLibraryScanProgressPayload = nil
     }
+
+    private func parseJsonPayload(_ payload: String) -> Any {
+        guard let data = payload.data(using: .utf8) else {
+            return payload
+        }
+        do {
+            return try JSONSerialization.jsonObject(with: data, options: [.fragmentsAllowed])
+        } catch {
+            return payload
+        }
+    }
     
     private func handleMethodCall(call: FlutterMethodCall, result: @escaping FlutterResult) {
         DispatchQueue.global(qos: .userInitiated).async {
@@ -215,11 +226,11 @@ import Gobackend  // Import Go framework
 
         case "getDownloadProgress":
             let response = GobackendGetDownloadProgress()
-            return response
+            return parseJsonPayload(response as String? ?? "{}")
             
         case "getAllDownloadProgress":
             let response = GobackendGetAllDownloadProgress()
-            return response
+            return parseJsonPayload(response as String? ?? "{}")
             
         case "initItemProgress":
             let args = call.arguments as! [String: Any]
@@ -979,7 +990,7 @@ import Gobackend  // Import Go framework
             
         case "getLibraryScanProgress":
             let response = GobackendGetLibraryScanProgressJSON()
-            return response
+            return parseJsonPayload(response as String? ?? "{}")
             
         case "cancelLibraryScan":
             GobackendCancelLibraryScanJSON()