Checkpoint — The Layered Checkout

The gate (mini-build)

Three seams in one file, checked by the tests through the ports only:

1. Ports — ICatalog { int Price(string sku); } and IBasket { void Add(string sku, int qty); int Count { get; } string FirstSku { get; } } (Count = total quantity added; FirstSku = the first sku ever added).
2. Service — Checkout(ICatalog, IBasket): Quote() returns "Total:" + price-of-FirstSku + " x " + Count; throws InvalidOperationException on an empty basket (Count == 0). Quote must work through the interfaces only — no casting to concrete types.
3. Batch boundary — QuoteMany(string[] skus, ICatalog catalog) returns one price per sku (input order), pricing each distinct sku exactly once — dedup, then map.

public static class Solution
{
    public interface ICatalog { int Price(string sku); }
    public sealed class MemCatalog : ICatalog;    // "t1" -> 25, "t2" -> 40
    public interface IBasket { void Add(string sku, int qty); int Count { get; } string FirstSku { get; } }
    public sealed class MemBasket : IBasket;
    public sealed class Checkout { public Checkout(ICatalog c, IBasket b); public string Quote(); }
    public static int[] QuoteMany(string[] skus, ICatalog catalog);
    public static bool Threw(Action a);
}