```

feat(AI): 集成MNN推理引擎替换MLX作为主AI运行时

- 引入MNN(alibaba) + Arm SME2 + CPU作为主AI运行时，支持A19/iPhone17的
  SME2和A17的NEON加速
- 添加MLX Swift作为兜底GPU推理方案，实现双后端切换机制
- 使用单一Qwen3.5-2B多模态模型(1.2GB)，替代原有的LLM+VL分离架构
- 实现InferenceEngine.current引擎选择逻辑，真机默认MNN，模拟器回退MLX
- 更新AIAgent架构，通过MNNLLMBridge(ObjC++) → MNNBackend进行推理
- 修改队列机制防止并发推理导致OOM，使用信号量闸门控制显存占用
- 更新文档中的技术栈说明、模块边界和周次交付计划
```

康康/Persistence/FileVault.swift

@@ -1,5 +1,6 @@
 import Foundation
 import UIKit
+import ImageIO
 
 enum FileVaultError: Error {
     case readFailed
@@ -10,7 +11,10 @@ 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。实例方法显式 `nonisolated`,见 ModelStore 同款注释。
-final class FileVault: @unchecked Sendable {
+/// 类级 `nonisolated`:工程开了 `SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor`,默认会把本类
+/// 连同 `thumbnailCache`(非 Sendable 的 NSCache)推成 MainActor,令 nonisolated I/O 方法 / 单例
+/// 初始化访问不了;本类是纯文件 I/O + 缓存工具,必须脱离 MainActor 供任意 actor 调用,故整类标 nonisolated。
+nonisolated final class FileVault: @unchecked Sendable {
     nonisolated static let shared: FileVault = {
         do {
             let appSupport = try FileManager.default.url(
@@ -28,6 +32,17 @@ final class FileVault: @unchecked Sendable {
 
     let rootURL: URL
 
+    /// 已降采样图片的内存缓存。NSCache 线程安全、内存吃紧时系统自动回收;
+    /// key = "相对路径@目标像素",避免 TabView 翻页 / 列表滚动反复读盘解码同一张图。
+    /// 只缓存降采样后的小图(几百 KB),不缓存全分辨率原图。
+    /// `nonisolated(unsafe)`:本工程默认 MainActor 隔离,非 Sendable 的 NSCache 存储属性即便整类标
+    /// nonisolated 仍被推成 MainActor,令各 nonisolated I/O 方法访问不到;NSCache 本身线程安全,故 unsafe 豁免。
+    private nonisolated(unsafe) let thumbnailCache: NSCache<NSString, UIImage> = {
+        let cache = NSCache<NSString, UIImage>()
+        cache.countLimit = 40
+        return cache
+    }()
+
     init(rootURL: URL) throws {
         self.rootURL = rootURL
         try FileManager.default.createDirectory(
@@ -81,6 +96,33 @@ final class FileVault: @unchecked Sendable {
         return image
     }
 
+    /// 按目标最大边降采样加载。用 ImageIO 直接解出缩略图,**绝不**把全分辨率位图载进内存:
+    /// 一张 4000×3000 体检照全量解码是 ~48MB RGBA,翻几页就 jetsam;降到 ≤2000px 后仅几 MB。
+    /// 自动尊重 EXIF 方向。结果按尺寸缓存,翻页/滚动回看命中缓存不再读盘。
+    /// 失败(锁屏读不到 / 损坏)抛 readFailed,与 loadImage 一致,UI 显示占位即可。
+    nonisolated func loadDownsampledImage(relativePath: String, maxPixelSize: CGFloat) throws -> UIImage {
+        let cacheKey = "\(relativePath)@\(Int(maxPixelSize))" as NSString
+        if let cached = thumbnailCache.object(forKey: cacheKey) { return cached }
+
+        let url = try resolveSafePath(relativePath)
+        let srcOptions: [CFString: Any] = [kCGImageSourceShouldCache: false]
+        guard let src = CGImageSourceCreateWithURL(url as CFURL, srcOptions as CFDictionary) else {
+            throw FileVaultError.readFailed
+        }
+        let thumbOptions: [CFString: Any] = [
+            kCGImageSourceCreateThumbnailFromImageAlways: true,
+            kCGImageSourceCreateThumbnailWithTransform: true,   // 应用 EXIF 旋转,免得横竖颠倒
+            kCGImageSourceShouldCacheImmediately: true,          // 在后台线程就解码,不拖到主线程绘制时
+            kCGImageSourceThumbnailMaxPixelSize: maxPixelSize
+        ]
+        guard let cg = CGImageSourceCreateThumbnailAtIndex(src, 0, thumbOptions as CFDictionary) else {
+            throw FileVaultError.decodeFailed
+        }
+        let image = UIImage(cgImage: cg)
+        thumbnailCache.setObject(image, forKey: cacheKey)
+        return image
+    }
+
     nonisolated func remove(relativePath: String) throws {
         let url = try resolveSafePath(relativePath)
         do {
@@ -88,6 +130,8 @@ final class FileVault: @unchecked Sendable {
         } catch {
             throw FileVaultError.removeFailed
         }
+        // 删文件后清掉降采样缓存,避免详情页仍显示已删原图(删除次数极少,整清无虞)。
+        thumbnailCache.removeAllObjects()
     }
 
     /// 清空 Vault 全部文件。单个文件删除失败(被占用/权限)不中断,继续删其余;
@@ -99,6 +143,7 @@ final class FileVault: @unchecked Sendable {
             try? fm.removeItem(at: url)
         }
         let remaining = (try? fm.contentsOfDirectory(at: rootURL, includingPropertiesForKeys: nil)) ?? []
+        thumbnailCache.removeAllObjects()
         if !remaining.isEmpty {
             throw FileVaultError.removeFailed
         }