feat(app): wire plugin host, ship smoke plugin, log lifecycle

Phase 1 MVP end-to-end. Program.cs initializes Serilog, builds an
AppPluginHost that hands plugins a SerilogAdapter (IPluginLogger),
discovers plugins from the App's output plugins/ dir, loads each via
PluginLoader, calls Enable on all of them before opening the GameWindow,
and calls Disable in a finally block on shutdown.

AcDream.Plugins.Smoke is a new first-party plugin that logs through
the host during Initialize / Enable / Disable. Its csproj references
the abstractions with Private=false + ExcludeAssets=runtime to avoid
shipping a second copy of AcDream.Plugin.Abstractions.dll (which would
break ALC type identity). An MSBuild Target on the App project copies
the plugin DLL into plugins/AcDream.Plugins.Smoke/ and writes the
plugin.json manifest next to it.

Smoke verified against real dats. Console output observed:
  [INF] scanning plugins in ...\plugins
  [INF] smoke plugin initialized
  [INF] loaded plugin acdream.smoke (Smoke Plugin)
  [INF] smoke plugin enabled
  loaded landblock 0xA9B4FFFF
  <window renders terrain>
  [INF] smoke plugin disabled  (on shutdown)

Phase 1 done.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.Plugins.Smoke/AcDream.Plugins.Smoke.csproj

@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+    <!-- Plugin DLLs are copied to plugins/<id>/ at build of AcDream.App.
+         They must NOT bring AcDream.Plugin.Abstractions.dll with them;
+         the host already owns it. -->
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\AcDream.Plugin.Abstractions\AcDream.Plugin.Abstractions.csproj">
+      <Private>false</Private>
+      <ExcludeAssets>runtime</ExcludeAssets>
+    </ProjectReference>
+  </ItemGroup>
+</Project>

src/AcDream.Plugins.Smoke/SmokePlugin.cs

@@ -0,0 +1,17 @@
+using AcDream.Plugin.Abstractions;
+
+namespace AcDream.Plugins.Smoke;
+
+public sealed class SmokePlugin : IAcDreamPlugin
+{
+    private IPluginHost? _host;
+
+    public void Initialize(IPluginHost host)
+    {
+        _host = host;
+        _host.Log.Info("smoke plugin initialized");
+    }
+
+    public void Enable() => _host?.Log.Info("smoke plugin enabled");
+    public void Disable() => _host?.Log.Info("smoke plugin disabled");
+}