Milestone 3 — The Authorized Service Layer

The engine's gates, in one service. Given provided user/project registries (test seeds them through Solution.Register/Solution.AddProject), implement: ``csharp public static class Solution { public static void Register(int id, string name, Role role); public static void AddProject(int id, string name, int ownerId); public static int CreateTask(int callerId, int projectId, string title, Priority priority); public static void Assign(int callerId, int taskId, int assigneeId); public static TaskItem? FindTask(int taskId); } ` Both types nest inside Solution. Gates, in order: 1. **valid input** — title via DomainGuard.ValidTitle, priority defined; bad → System.ArgumentException, 2. **allowed** — CreateTask requires the caller to be the project's owner or an Admin; Assign requires the caller to be the task's project owner or an Admin. Not allowed → System.InvalidOperationException, 3. **possible** — unknown project/task/assignee/user → System.Collections.Generic.KeyNotFoundException, 4. success — CreateTask stores a Todo task (via your own repo logic or a plain list — your choice, IDs 1,2,3…) and returns its id; Assign sets the assignee; FindTask returns the item or null`.