fix(concurrency): nonisolated(unsafe) static shared + 修同 actor 内冗余 await

项目开启了 -default-isolation=MainActor upcoming feature,导致:

1. static let shared 默认被视为 MainActor 隔离,即使 class 标了
   @unchecked Sendable,从其他 actor(如 AIRuntime)同步访问仍报
   "Expression is 'async' but is not marked with 'await'".

   修法:ModelStore.shared 和 FileVault.shared 都加 nonisolated(unsafe)
   修饰,明确"任何隔离上下文都可同步访问"。

2. AIRuntime.generate() 内的 Task { ... } 继承 AIRuntime actor 隔离,
   self.recordRate 是同 actor 内部调用,不需要 await,否则报
   "No 'async' operations occur within 'await' expression".

   修法:去掉冗余的 await。

** BUILD SUCCEEDED ** 已验证。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

体己/Persistence/FileVault.swift

@@ -10,8 +10,9 @@ 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。
 final class FileVault: @unchecked Sendable {
-    static let shared: FileVault = {
+    nonisolated(unsafe) static let shared: FileVault = {
         do {
             let appSupport = try FileManager.default.url(
                 for: .applicationSupportDirectory,