chore: core 多模块骨架

IosClientKit/Sources/CoreDatabase/PersistenceController.swift

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+import Foundation
+import SwiftData
+
+/// 打卡习惯的持久化实体（对应 Android Room 的 @Entity）。
+@Model
+public final class HabitEntity {
+    public var id: String
+    public var title: String
+    public var detail: String
+    public var streakDays: Int
+    public var createdAt: Date
+
+    public init(id: String, title: String, detail: String, streakDays: Int, createdAt: Date) {
+        self.id = id
+        self.title = title
+        self.detail = detail
+        self.streakDays = streakDays
+        self.createdAt = createdAt
+    }
+}
+
+/// SwiftData 容器工厂（对应 Android RoomDatabase）。
+public enum PersistenceController {
+    public static func makeModelContainer(inMemory: Bool = false) throws -> ModelContainer {
+        let configuration = ModelConfiguration(isStoredInMemoryOnly: inMemory)
+        return try ModelContainer(for: HabitEntity.self, configurations: configuration)
+    }
+}