feat: 定义仓库协议与用例（含会话）

IosClientKit/Sources/CoreDomain/CheckinUseCase.swift

@@ -0,0 +1,18 @@
+import CoreModel
+
+/// 获取/提交打卡数据的用例。
+public struct CheckinUseCase: Sendable {
+    private let repository: CheckinRepository
+
+    public init(repository: CheckinRepository) {
+        self.repository = repository
+    }
+
+    public func observeCheckin() -> AsyncThrowingStream<CheckinRecord, Error> {
+        repository.observeCheckin()
+    }
+
+    public func submitCheckin(status: CheckinStatus) async throws {
+        try await repository.submitCheckin(status: status)
+    }
+}