Scaffold the .sln and M0 project structure

Buildable skeleton matching docs/kernel-contract.md:

- src/Engine.Kernel — the frozen kernel. Interfaces and data types only
  (World, Component, GameObject, Transform, ISchedule, IServiceRegistry,
  IEventBus, ILogger, IPlugin/IPluginContext, plugin.json and project.json
  manifest models). No PluginHost/Scheduler/World implementation yet —
  that's M0's actual work, not scaffolding.
- src/Engine.Host — the CLI runtime entry point, placeholder for now.
- plugins/sandbox.echo — a minimal two-assembly plugin (Contracts in
  Default ALC, implementation in collectible ALC) that exists only to
  exercise the reload loop end to end once PluginHost exists.
- tests/Engine.Kernel.Tests, tests/Engine.ConformanceHarness — wired up
  with one Skip-marked placeholder test each, naming what M0 needs to
  make them real (including the 200-cycle ALC leak test from §4). The
  harness references the sandbox plugin with
  ReferenceOutputAssembly="false" so it loads it dynamically by path
  instead of linking its types into its own Default ALC.
- samples/EmptyProject — a starter project.json.
- Directory.Build.props centralizes shared settings across every
  project, including AllowUnsafeBlocks=false to enforce the §7 rule at
  the build level rather than by convention.

Solution builds clean and `dotnet test` runs both placeholders as
Skipped (not failing) — confirms the wiring, not the engine.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N1qPfzq8TDCUMFMV3UwV5N

src/Engine.Kernel/World/GameObject.cs

@@ -0,0 +1,28 @@
+namespace Engine.Kernel.World;
+
+/// <summary>
+/// The entity type: identity, hierarchy, and a list of components. See
+/// docs/kernel-contract.md §2.
+/// </summary>
+public sealed class GameObject
+{
+    public required string Name { get; set; }
+
+    public Transform Transform;
+
+    public GameObject? Parent { get; internal set; }
+
+    // TODO(M0): backing storage for children, tags, and the component list
+    // + type index that makes World.Query<T>() O(matches), not O(all).
+
+    public IReadOnlyList<GameObject> Children => throw new NotImplementedException();
+
+    public T? GetComponent<T>() where T : Component
+        => throw new NotImplementedException();
+
+    public T AddComponent<T>() where T : Component, new()
+        => throw new NotImplementedException();
+
+    public void RemoveComponent<T>() where T : Component
+        => throw new NotImplementedException();
+}