ContextKit

```swift
// MARK: - Data Model
struct UserProfile: Codable, Identifiable {
    let id: UUID
    var name: String
    var email: String
    var joinDate: Date
    var isPremium: Bool

    // Business logic for premium status
    var membershipStatus: String {
        isPremium ? "Active Premium" : "Free Tier"
    }
}

// MARK: - ViewModel
class UserProfileViewModel: ObservableObject {
    @Published var user: UserProfile
    @Published var isLoading = false
    @Published var errorMessage: String?

    init(user: UserProfile) {
        self.user = user
    }

    func updateEmail(newEmail: String) async throws {
        guard newEmail.contains("@") else {
            throw ValidationError.invalidEmail
        }
        isLoading = true
        try await Task.sleep(nanoseconds: 1_000_000_000) // Simulate API call
        user.email = newEmail
        isLoading = false
    }

    enum ValidationError: Error {
        case invalidEmail
    }
}

// MARK: - SwiftUI View
struct UserProfileView: View {
    @StateObject var viewModel: UserProfileViewModel

    var body: some View {
        Form {
            Section(header: Text("Profile Information")) {
                TextField("Name", text: Binding(
                    get: { viewModel.user.name },
                    set: { viewModel.user.name = $0 }
                ))
                TextField("Email", text: Binding(
                    get: { viewModel.user.email },
                    set: { _ in /* Handled by ViewModel */ }
                ))
                Toggle("Premium Member", isOn: Binding(
                    get: { viewModel.user.isPremium },
                    set: { viewModel.user.isPremium = $0 }
                ))
            }

            Section {
                if viewModel.isLoading {
                    ProgressView()
                } else if let error = viewModel.errorMessage {
                    Text(error)
                        .foregroundColor(.red)
                }
            }
        }
        .navigationTitle("User Profile")
    }
}

// MARK: - Unit Tests
import XCTest
@testable import MyApp

class UserProfileTests: XCTestCase {
    func testEmailValidation() {
        let viewModel = UserProfileViewModel(user: UserProfile(id: UUID(), name: "Test", email: "test@example.com", joinDate: Date(), isPremium: false))
        XCTAssertNoThrow(try viewModel.updateEmail(newEmail: "valid@example.com"))
        XCTAssertThrowsError(try viewModel.updateEmail(newEmail: "invalid-email"))
    }

    func testPremiumStatus() {
        let premiumUser = UserProfile(id: UUID(), name: "Premium", email: "premium@example.com", joinDate: Date(), isPremium: true)
        XCTAssertEqual(premiumUser.membershipStatus, "Active Premium")
        
        let freeUser = UserProfile(id: UUID(), name: "Free", email: "free@example.com", joinDate: Date(), isPremium: false)
        XCTAssertEqual(freeUser.membershipStatus, "Free Tier")
    }
}
```

**File Structure Generated:**
```
Sources/
├── Models/
│   └── UserProfile.swift
├── ViewModels/
│   └── UserProfileViewModel.swift
├── Views/
│   └── UserProfileView.swift
└── Tests/
    └── UserProfileTests.swift
```

**Assumptions:**
1. Using MVVM architecture with Combine for state management
2. iOS 15+ deployment target
3. Swift 5.7+ with async/await support
4. Existing UserProfile model is not present (created new)
5. No existing networking layer (local state only for this example)

**Next Steps:**
1. Add network layer integration in ViewModel
2. Implement persistence using CoreData or UserDefaults
3. Add accessibility identifiers for UI testing
4. Localize strings for internationalization