Checkpoint — The Wired Engine

The gate (mini-build)

Two seams, final form:

1. JSON snapshot round-trip — implement Solution.Snapshot(System.Collections.Generic.IReadOnlyList<TaskItem> tasks) returning the JSON string for the list (camelCase property names, enums as strings), and Solution.Load(string json) parsing it back with case-insensitive names. Round-trip must preserve id, projectId, title, assigneeId, priority, and status exactly (order included).
2. Wiring — implement Solution.BuildEngine(FakeStore store) that constructs the engine as the composition root would: a Service over the given store, returning it. The Service (nested in Solution, constructor takes FakeStore) must expose System.Threading.Tasks.Task<int> CreateAsync(int projectId, string title, Priority priority, System.Threading.CancellationToken ct) — validates the title (DomainGuard.ValidTitle), allocates ids 1,2,3…, stores via the store's Task PutAsync(TaskItem item, System.Threading.CancellationToken ct), respects cancellation (a cancelled caller gets System.OperationCanceledException and nothing is stored), surfaces store failures to its caller, and returns the new id. FakeStore (provided) records puts and can be pre-seeded with Seed(TaskItem); its Task PutAsync completes synchronously (a fake), and Puts lists what arrived.

public static class Solution
{
    public static string Snapshot(System.Collections.Generic.IReadOnlyList<TaskItem> tasks);
    public static TaskItem[] Load(string json);
    public sealed class Service
    {
        public Service(FakeStore store);
        public System.Threading.Tasks.Task<int> CreateAsync(int projectId, string title, Priority priority, System.Threading.CancellationToken ct);
    }
    public static Service BuildEngine(FakeStore store);
}
// provided: TaskItem, Priority, Status, DomainGuard.ValidTitle,
// FakeStore { Task PutAsync(TaskItem, CancellationToken); List<TaskItem> Puts; void Seed(TaskItem); }