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/Scheduling/ISchedule.cs

@@ -0,0 +1,13 @@
+namespace Engine.Kernel.Scheduling;
+
+/// <summary>
+/// Frame stages and system ordering. See docs/kernel-contract.md §2.
+/// </summary>
+public interface ISchedule
+{
+    ISystemBuilder Add(Stage stage, Delegate system);
+
+    /// <summary>Called from a plugin's Shutdown() — must remove everything
+    /// Configure() added, or the ALC it lives in will never unload. See §4.</summary>
+    void RemoveAllFrom(string pluginId);
+}

src/Engine.Kernel/Scheduling/ISystemBuilder.cs

@@ -0,0 +1,16 @@
+using Engine.Kernel.World;
+
+namespace Engine.Kernel.Scheduling;
+
+/// <summary>
+/// Fluent builder returned by <see cref="ISchedule.Add"/>. Declared
+/// Reads/Writes are what lets the scheduler run systems with disjoint
+/// access in parallel and, in debug builds, enforce that a system only
+/// touches what it declared. See docs/kernel-contract.md §2 and §7.
+/// </summary>
+public interface ISystemBuilder
+{
+    ISystemBuilder After(string systemId);
+    ISystemBuilder Reads<T>() where T : Component;
+    ISystemBuilder Writes<T>() where T : Component;
+}

src/Engine.Kernel/Scheduling/Stage.cs

@@ -0,0 +1,12 @@
+namespace Engine.Kernel.Scheduling;
+
+/// <summary>
+/// Open question (see the footer of docs/kernel-contract.md): whether the
+/// set of stages is fixed or plugin-extensible. This is only enough to
+/// make the §3 example compile — not a decision.
+/// </summary>
+public enum Stage
+{
+    Update,
+    Render,
+}