harden(ai): ModelStore seedFromBundle 在 DEBUG 报错,加空目录测试

按 code quality review 反馈:
- seedFromBundle 找不到 bundle 资源时,DEBUG 下 assertionFailure 提示
  target membership(release 仍静默 return),避免 W6 启用时排查困难
- 补 totalBytesReturnsZeroWhenFolderMissing 测试,覆盖 folder 不存在时
  enumerator 为 nil 的 guard 路径

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

体己/AI/ModelStore.swift

@@ -65,6 +65,9 @@ final class ModelStore {
     /// Demo 现场旁路:从 Bundle 拷贝预装模型(W6 才真正使用,本周占位)
     func seedFromBundle(_ kind: ModelKind) throws {
         guard let bundleURL = Bundle.main.url(forResource: kind.rawValue, withExtension: nil) else {
+            #if DEBUG
+            assertionFailure("Bundle 缺少 \(kind.rawValue),检查资源是否加入 target")
+            #endif
             return
         }
         let target = localURL(for: kind)