fix(IOS-INFO-043): set vm.filePath after COS download + cleanup

- MaterialReaderViewModel.filePath: let → var for runtime updates
- Set vm.filePath to downloaded temp path before vm.load()
- Remove unused downloadedFilePath/resolvedPath

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AIStudyApp/AIStudyApp/Features/MaterialReader/MaterialReaderView.swift

@@ -24,7 +24,7 @@ final class MaterialReaderViewModel: ObservableObject {
     @Published var stats: TextStats?
 
     let materialId: String
-    let filePath: String
+    var filePath: String
     let materialType: MaterialType
     let mode: PreviewMode
 
@@ -109,19 +109,9 @@ struct MaterialReaderView: View {
     @Environment(\.scenePhase) private var scenePhase
     @State private var isParsing = false
     @State private var parseMessage = ""
-    @State private var downloadedFilePath: String?
-
     private let title: String
     private let knowledgeBaseId: String?
 
-    /// Resolved file path: uses downloaded temp file if original is empty/missing
-    private var resolvedPath: String {
-        if !vm.filePath.isEmpty && FileManager.default.fileExists(atPath: vm.filePath) {
-            return vm.filePath
-        }
-        return downloadedFilePath ?? vm.filePath
-    }
-
     // V2 session manager (primary) + V1 collector (fallback)
     private let sessionManager = ReadingRuntimeSessionManager.shared
     private let collector = ReadingEventCollector.shared
@@ -262,8 +252,8 @@ struct MaterialReaderView: View {
                             let localURL = FileManager.default.temporaryDirectory
                                 .appendingPathComponent("source_\(vm.materialId).\(ext)")
                             try data.write(to: localURL)
-                            downloadedFilePath = localURL.path
                             print("[MaterialReader] Downloaded to: \(localURL.path), size: \(data.count)")
+                            vm.filePath = localURL.path
                         }
                     } catch {
                         print("[MaterialReader] COS download failed: \(error.localizedDescription)")