harden(persistence): FileVault path traversal guard + error unification

按 code quality review 反馈(P0 + 4×P1):
- 加 resolveSafePath() 拒绝 / 和 .. 并验证 hasPrefix(rootURL)
- loadImage/remove 统一抛 FileVaultError(readFailed/removeFailed)
- 删除测试 struct 上多余的 @MainActor
- 每个 @Test 加 defer cleanup,不泄漏 temp 目录
- 测试图片改用生成 16x16 红色,不依赖 SF Symbol

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

体己Tests/FileVaultTests.swift

@@ -2,7 +2,6 @@ import Testing
 import UIKit
 @testable import 体己
 
-@MainActor
 struct FileVaultTests {
 
     private func makeIsolatedVault() throws -> FileVault {
@@ -11,9 +10,19 @@ struct FileVaultTests {
         return try FileVault(rootURL: temp)
     }
 
+    private func makeTestImage() -> UIImage {
+        let size = CGSize(width: 16, height: 16)
+        let renderer = UIGraphicsImageRenderer(size: size)
+        return renderer.image { ctx in
+            UIColor.red.setFill()
+            ctx.fill(CGRect(origin: .zero, size: size))
+        }
+    }
+
     @Test func writeAndReadJPEGRoundtrip() throws {
         let vault = try makeIsolatedVault()
-        let image = UIImage(systemName: "doc")!
+        defer { try? FileManager.default.removeItem(at: vault.rootURL) }
+        let image = makeTestImage()
 
         let saved = try vault.writeJPEG(image, quality: 0.8)
 
@@ -26,7 +35,8 @@ struct FileVaultTests {
 
     @Test func removeMakesFileGone() throws {
         let vault = try makeIsolatedVault()
-        let saved = try vault.writeJPEG(UIImage(systemName: "doc")!)
+        defer { try? FileManager.default.removeItem(at: vault.rootURL) }
+        let saved = try vault.writeJPEG(makeTestImage())
 
         try vault.remove(relativePath: saved.relativePath)
 
@@ -37,8 +47,9 @@ struct FileVaultTests {
 
     @Test func wipeRemovesAllFiles() throws {
         let vault = try makeIsolatedVault()
-        let a = try vault.writeJPEG(UIImage(systemName: "doc")!)
-        let b = try vault.writeJPEG(UIImage(systemName: "folder")!)
+        defer { try? FileManager.default.removeItem(at: vault.rootURL) }
+        let a = try vault.writeJPEG(makeTestImage())
+        let b = try vault.writeJPEG(makeTestImage())
 
         try vault.wipe()