fix(concurrency): clear 4 Swift 6 warnings under default MainActor isolation

- ModelStore/FileVault: drop nonisolated(unsafe) on shared, mark all instance
  methods nonisolated (they only read filesystem); ModelKind enum also nonisolated
- AIRuntime ↔ ModelStore cross-actor call resolved by the above
- LLMSession: replace deprecated Device.setDefault(device:) with task-scoped
  Device.withDefaultDevice(.cpu, body:); wrap both load and generate so the
  TaskLocal propagates through ModelContainer.perform

康康/Persistence/FileVault.swift

@@ -9,10 +9,9 @@ enum FileVaultError: Error {
 }
 
 /// `@unchecked Sendable`:rootURL 是 let,方法只 I/O 到沙盒目录(线程安全),
-/// 可被任意 actor / Task 跨边界访问。
-/// `nonisolated(unsafe) shared`:见 ModelStore 同款注释。
+/// 可被任意 actor / Task 跨边界访问。实例方法显式 `nonisolated`,见 ModelStore 同款注释。
 final class FileVault: @unchecked Sendable {
-    nonisolated(unsafe) static let shared: FileVault = {
+    nonisolated static let shared: FileVault = {
         do {
             let appSupport = try FileManager.default.url(
                 for: .applicationSupportDirectory,
@@ -45,7 +44,7 @@ final class FileVault: @unchecked Sendable {
 
     // MARK: - Path Safety
 
-    private func resolveSafePath(_ relativePath: String) throws -> URL {
+    nonisolated private func resolveSafePath(_ relativePath: String) throws -> URL {
         guard !relativePath.contains("/"),
               !relativePath.contains(".."),
               !relativePath.isEmpty else {
@@ -60,7 +59,7 @@ final class FileVault: @unchecked Sendable {
 
     // MARK: - Public API
 
-    func writeJPEG(_ image: UIImage, quality: CGFloat = 0.85) throws -> SavedAsset {
+    nonisolated func writeJPEG(_ image: UIImage, quality: CGFloat = 0.85) throws -> SavedAsset {
         guard let data = image.jpegData(compressionQuality: quality) else {
             throw FileVaultError.writeFailed
         }
@@ -70,7 +69,7 @@ final class FileVault: @unchecked Sendable {
         return SavedAsset(relativePath: filename, bytes: data.count)
     }
 
-    func loadImage(relativePath: String) throws -> UIImage {
+    nonisolated func loadImage(relativePath: String) throws -> UIImage {
         let url = try resolveSafePath(relativePath)
         let data: Data
         do {
@@ -82,7 +81,7 @@ final class FileVault: @unchecked Sendable {
         return image
     }
 
-    func remove(relativePath: String) throws {
+    nonisolated func remove(relativePath: String) throws {
         let url = try resolveSafePath(relativePath)
         do {
             try FileManager.default.removeItem(at: url)
@@ -91,7 +90,7 @@ final class FileVault: @unchecked Sendable {
         }
     }
 
-    func wipe() throws {
+    nonisolated func wipe() throws {
         let fm = FileManager.default
         let contents = try fm.contentsOfDirectory(at: rootURL, includingPropertiesForKeys: nil)
         for url in contents {