feat(persistence): add FileVault with complete file protection

按 W2 plan Task 3 落地原图加密存储:
- writeJPEG / loadImage / remove / wipe 四个核心操作
- Application Support/Vault/ 目录全程 .completeFileProtection
- 文件写入用 .completeFileProtection options(双保险)
- FileVault(rootURL:) 注入便于测试隔离
- shared 单例用真实 App Support 路径

测试 3 条:roundtrip / remove / wipe。

注:.swift 文件需用户在 Xcode 拖入 target(Persistence group + 体己Tests),
之后 ⌘U 跑测试,若全绿再 amend 提交 .pbxproj。

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

体己/Persistence/FileVault.swift

@@ -0,0 +1,72 @@
+import Foundation
+import UIKit
+
+enum FileVaultError: Error {
+    case readFailed
+    case writeFailed
+    case removeFailed
+    case decodeFailed
+}
+
+final class FileVault {
+    static let shared: FileVault = {
+        do {
+            let appSupport = try FileManager.default.url(
+                for: .applicationSupportDirectory,
+                in: .userDomainMask,
+                appropriateFor: nil,
+                create: true
+            )
+            let vaultURL = appSupport.appendingPathComponent("Vault", isDirectory: true)
+            return try FileVault(rootURL: vaultURL)
+        } catch {
+            fatalError("FileVault.shared init failed: \(error)")
+        }
+    }()
+
+    let rootURL: URL
+
+    init(rootURL: URL) throws {
+        self.rootURL = rootURL
+        try FileManager.default.createDirectory(
+            at: rootURL,
+            withIntermediateDirectories: true,
+            attributes: [.protectionKey: FileProtectionType.complete]
+        )
+    }
+
+    struct SavedAsset {
+        let relativePath: String
+        let bytes: Int
+    }
+
+    func writeJPEG(_ image: UIImage, quality: CGFloat = 0.85) throws -> SavedAsset {
+        guard let data = image.jpegData(compressionQuality: quality) else {
+            throw FileVaultError.writeFailed
+        }
+        let filename = "\(UUID().uuidString).jpg"
+        let url = rootURL.appendingPathComponent(filename)
+        try data.write(to: url, options: [.atomic, .completeFileProtection])
+        return SavedAsset(relativePath: filename, bytes: data.count)
+    }
+
+    func loadImage(relativePath: String) throws -> UIImage {
+        let url = rootURL.appendingPathComponent(relativePath)
+        let data = try Data(contentsOf: url)
+        guard let image = UIImage(data: data) else { throw FileVaultError.decodeFailed }
+        return image
+    }
+
+    func remove(relativePath: String) throws {
+        let url = rootURL.appendingPathComponent(relativePath)
+        try FileManager.default.removeItem(at: url)
+    }
+
+    func wipe() throws {
+        let fm = FileManager.default
+        let contents = try fm.contentsOfDirectory(at: rootURL, includingPropertiesForKeys: nil)
+        for url in contents {
+            try fm.removeItem(at: url)
+        }
+    }
+}