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

plugins/sandbox.echo/Sandbox.Echo.Contracts/Ping.cs

@@ -0,0 +1,13 @@
+using Engine.Kernel.World;
+
+namespace Sandbox.Echo.Contracts;
+
+/// <summary>
+/// Minimal component for exercising the reload loop described in
+/// docs/kernel-contract.md §7 — not a real engine feature, just something
+/// for a system to touch before any real plugin exists.
+/// </summary>
+public sealed class Ping : Component
+{
+    public int Count;
+}

plugins/sandbox.echo/Sandbox.Echo.Contracts/Sandbox.Echo.Contracts.csproj

@@ -0,0 +1,7 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\..\src\Engine.Kernel\Engine.Kernel.csproj" />
+  </ItemGroup>
+
+</Project>

plugins/sandbox.echo/Sandbox.Echo/EchoPlugin.cs

@@ -0,0 +1,33 @@
+using Engine.Kernel.Plugins;
+using Engine.Kernel.Scheduling;
+using Engine.Kernel.World;
+using Sandbox.Echo.Contracts;
+
+namespace Sandbox.Echo;
+
+/// <summary>
+/// M0's test fixture. Exists only to exercise the reload loop end to end —
+/// edit, rebuild, ALC reload, headless run, dump — before any real
+/// subsystem exists to test it against. See M0 in docs/kernel-contract.md §8.
+/// </summary>
+public sealed class EchoPlugin : IPlugin
+{
+    public void Configure(IPluginContext ctx)
+    {
+        ctx.Schedule.Add(Stage.Update, Tick)
+           .Writes<Ping>();
+
+        ctx.Log.Info("sandbox.echo configured");
+    }
+
+    public void Shutdown(IPluginContext ctx)
+    {
+        ctx.Schedule.RemoveAllFrom("sandbox.echo");
+    }
+
+    static void Tick(IWorld world)
+    {
+        // TODO(M0): bump every Ping.Count by one. Placeholder until the
+        // Scheduler actually exists to invoke this.
+    }
+}

plugins/sandbox.echo/Sandbox.Echo/Sandbox.Echo.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\..\src\Engine.Kernel\Engine.Kernel.csproj" />
+    <ProjectReference Include="..\Sandbox.Echo.Contracts\Sandbox.Echo.Contracts.csproj" />
+  </ItemGroup>
+
+</Project>

plugins/sandbox.echo/plugin.json

@@ -0,0 +1,8 @@
+{
+  "id":        "sandbox.echo",
+  "version":   "0.0.1",
+  "contracts": "Sandbox.Echo.Contracts.dll",
+  "assembly":  "Sandbox.Echo.dll",
+  "dependsOn": {},
+  "reloadable": true
+}