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/AI/ModelStore.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-enum ModelKind: String, CaseIterable {
+nonisolated enum ModelKind: String, CaseIterable {
     /// 与 HuggingFace mlx-community 仓库名一一对应,也是沙盒 Models/ 下的子目录名。
     case llm = "Qwen3-1.7B-4bit"
     case vl  = "Qwen2.5-VL-3B-Instruct-4bit"
@@ -21,10 +21,10 @@ enum ModelKind: String, CaseIterable {
 
 /// `@unchecked Sendable`:rootURL 是 let,方法只读 filesystem(线程安全),
 /// 可被任意 actor / Task 跨边界访问。
-/// `nonisolated(unsafe) shared`:项目开启 `-default-isolation=MainActor` 后
-/// static 默认 MainActor 隔离,跨 actor 访问需要 await。这里手动 opt-out。
+/// 实例方法显式 `nonisolated`:项目开了 `SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor`,
+/// 默认会把方法推到 MainActor,导致从 `AIRuntime` 等 actor 调用报错。
 final class ModelStore: @unchecked Sendable {
-    nonisolated(unsafe) static let shared: ModelStore = {
+    nonisolated static let shared: ModelStore = {
         do {
             let appSupport = try FileManager.default.url(
                 for: .applicationSupportDirectory,
@@ -46,16 +46,16 @@ final class ModelStore: @unchecked Sendable {
         try FileManager.default.createDirectory(at: rootURL, withIntermediateDirectories: true)
     }
 
-    func localURL(for kind: ModelKind) -> URL {
+    nonisolated func localURL(for kind: ModelKind) -> URL {
         rootURL.appendingPathComponent(kind.rawValue, isDirectory: true)
     }
 
-    func isReady(_ kind: ModelKind) -> Bool {
+    nonisolated func isReady(_ kind: ModelKind) -> Bool {
         let sentinel = localURL(for: kind).appendingPathComponent(kind.sentinelFilename)
         return FileManager.default.fileExists(atPath: sentinel.path)
     }
 
-    func totalBytes(for kind: ModelKind) -> Int {
+    nonisolated func totalBytes(for kind: ModelKind) -> Int {
         let folder = localURL(for: kind)
         guard let enumerator = FileManager.default.enumerator(
             at: folder,
@@ -71,7 +71,7 @@ final class ModelStore: @unchecked Sendable {
     }
 
     /// Demo 现场旁路:从 Bundle 拷贝预装模型(W6 才真正使用,本周占位)
-    func seedFromBundle(_ kind: ModelKind) throws {
+    nonisolated func seedFromBundle(_ kind: ModelKind) throws {
         guard let bundleURL = Bundle.main.url(forResource: kind.rawValue, withExtension: nil) else {
             #if DEBUG
             assertionFailure("Bundle 缺少 \(kind.rawValue),检查资源是否加入 target")