Checkpoint — Composite Pipeline

The gate (mini-build)

One file, three seams, composing patterns you built this module:

1. Strategy — IRule { string? Check(Solution.Order o); } returning an error message or null. Implement MinQty (fails when total qty < 2, msg "qty") and PaidSku (fails when any sku is empty, msg "sku").
2. Composite/Strategy pipeline — Solution.Validate(order, IRule[] rules) returns the first non-null error, or null when all pass. Null rules in the array are skipped (tolerant pipeline).
3. Repository + audit Decorator — IRepo { void Save(Solution.Order o); int Saved; }; AuditedRepo wraps any IRepo, counts accepted saves in Saves, and the test flow decides what gets saved.

Flow under test: validate first; save only when valid; AuditedRepo.Saves must equal the number of valid orders.

public static class Solution
{
    public sealed class Order { public string Sku = ""; public int Qty; }
    public interface IRule { string? Check(Order o); }
    public static string? Validate(Order o, IRule[] rules);
    public interface IRepo { void Save(Order o); int Saved { get; } }
    public sealed class AuditedRepo : IRepo { public int Saves; /* wraps IRepo */ }
}