fix(ai,persistence): ModelStore + FileVault 标 @unchecked Sendable

Xcode 26 默认开启 Swift 6 严格并发检查。AIRuntime(actor)
调用 ModelStore.shared.isReady(...) 跨 actor 边界,因 ModelStore
非 Sendable 而编译报错"Expression is 'async' but is not marked
with 'await'; this is an error in the Swift 6 language mode"。

两个类的内部状态只读(rootURL: let),方法只做线程安全的
filesystem I/O,符合 Sendable 语义,标 @unchecked Sendable
即可,不必加锁或重构。

修复目标错误:
- AIRuntime.swift:48 - guard ModelStore.shared.isReady(.llm) ...
- 后续 CaptureService 调 FileVault.shared.writeJPEG 同样路径

不影响:
- HomeView/B5ResultView 里 Text "+" 的 macOS 26.0 deprecation 是
  warning,不阻塞 build,留待 UI polish 周清理

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

体己/Persistence/FileVault.swift

@@ -8,7 +8,9 @@ enum FileVaultError: Error {
     case decodeFailed
 }
 
-final class FileVault {
+/// `@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。
+final class FileVault: @unchecked Sendable {
     static let shared: FileVault = {
         do {
             let appSupport = try FileManager.default.url(