feat(platform): Campaign LA LA0 — extract ApplicationPathSet to AcDream.Platform

The launcher (LA3/LA4) needs the XDG/Windows path contract
(ApplicationPathSet/IApplicationPathEnvironment) without pulling in any
gameplay assembly. Move it out of AcDream.Runtime into a new BCL-only
AcDream.Platform project so the launcher-side Launcher.Core project can
reference it directly per the campaign plan (docs/plans/2026-08-14-launcher-campaign.md,
LA0). Namespace renamed AcDream.Runtime.Platform -> AcDream.Platform;
code is otherwise byte-identical (no logic changes).

AcDream.Runtime now carries a ProjectReference to AcDream.Platform and
re-exports it transitively, so App and Headless keep resolving the type
without a direct reference and K0's Headless single-ProjectReference
guard (HeadlessAssemblyReferencesOnlyTheRuntimeProject) stands unchanged.
The sibling Runtime dependency-boundary guard
(RuntimeProjectDeclaresOnlyApprovedProjectDependencies) does assert
Runtime's own project-reference set, so it needed a deliberate,
documented addition of AcDream.Platform to its expected list.

Moved tests/AcDream.Runtime.Tests/Platform/ApplicationPathSetTests.cs to
a new tests/AcDream.Platform.Tests/ project (namespace
AcDream.Platform.Tests) referencing only AcDream.Platform. Registered
both new projects in AcDream.slnx.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

AcDream.slnx

@@ -7,6 +7,7 @@
     <Project Path="src/AcDream.Core/AcDream.Core.csproj" />
     <Project Path="src/AcDream.Core.Net/AcDream.Core.Net.csproj" />
     <Project Path="src/AcDream.Headless/AcDream.Headless.csproj" />
+    <Project Path="src/AcDream.Platform/AcDream.Platform.csproj" />
     <Project Path="src/AcDream.Plugin.Abstractions/AcDream.Plugin.Abstractions.csproj" />
     <Project Path="src/AcDream.Plugins.Smoke/AcDream.Plugins.Smoke.csproj" />
     <Project Path="src/AcDream.Runtime/AcDream.Runtime.csproj" />
@@ -24,6 +25,7 @@
     <Project Path="tests/AcDream.Core.Tests/AcDream.Core.Tests.csproj" />
     <Project Path="tests/AcDream.Core.Net.Tests/AcDream.Core.Net.Tests.csproj" />
     <Project Path="tests/AcDream.Headless.Tests/AcDream.Headless.Tests.csproj" />
+    <Project Path="tests/AcDream.Platform.Tests/AcDream.Platform.Tests.csproj" />
     <Project Path="tests/AcDream.Runtime.Tests/AcDream.Runtime.Tests.csproj" />
     <Project Path="tests/AcDream.UI.Abstractions.Tests/AcDream.UI.Abstractions.Tests.csproj" />
   </Folder>

src/AcDream.App/Platform/GraphicalHostPlatformServices.cs

@@ -1,6 +1,6 @@
 using System.Runtime.InteropServices;
 using AcDream.App.Rendering;
-using AcDream.Runtime.Platform;
+using AcDream.Platform;
 
 namespace AcDream.App.Platform;
 

src/AcDream.App/Platform/GraphicalLegacyConfigurationMigrator.cs

@@ -1,4 +1,4 @@
-using AcDream.Runtime.Platform;
+using AcDream.Platform;
 
 namespace AcDream.App.Platform;
 

src/AcDream.App/Program.cs

@@ -3,7 +3,7 @@ using AcDream.App.Plugins;
 using AcDream.App.Platform;
 using AcDream.App.Rendering;
 using AcDream.Core.Plugins;
-using AcDream.Runtime.Platform;
+using AcDream.Platform;
 using Serilog;
 
 GraphicalHostPlatformServices graphicalPlatform =

src/AcDream.App/Rendering/GameWindow.cs

@@ -8,8 +8,8 @@ using AcDream.App.Settings;
 using AcDream.App.Platform;
 using AcDream.App.World;
 using AcDream.Content;
+using AcDream.Platform;
 using AcDream.Runtime;
-using AcDream.Runtime.Platform;
 using AcDream.Runtime.Entities;
 using AcDream.Runtime.Gameplay;
 using AcDream.Runtime.Session;

src/AcDream.Headless/Platform/HeadlessPathSet.cs

@@ -1,5 +1,5 @@
 using AcDream.Headless.Configuration;
-using AcDream.Runtime.Platform;
+using AcDream.Platform;
 
 namespace AcDream.Headless.Platform;
 

src/AcDream.Headless/Platform/HeadlessPlatformEnvironment.cs

@@ -1,4 +1,4 @@
-using AcDream.Runtime.Platform;
+using AcDream.Platform;
 
 namespace AcDream.Headless.Platform;
 

src/AcDream.Platform/AcDream.Platform.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+</Project>

src/AcDream.Platform/ApplicationPathSet.cs

@@ -1,4 +1,4 @@
-namespace AcDream.Runtime.Platform;
+namespace AcDream.Platform;
 
 /// <summary>
 /// BCL-only view of the process environment used to resolve portable

src/AcDream.Runtime/AcDream.Runtime.csproj

@@ -19,6 +19,7 @@
     <ProjectReference Include="..\AcDream.Core\AcDream.Core.csproj" />
     <ProjectReference Include="..\AcDream.Core.Net\AcDream.Core.Net.csproj" />
     <ProjectReference Include="..\AcDream.Content\AcDream.Content.csproj" />
+    <ProjectReference Include="..\AcDream.Platform\AcDream.Platform.csproj" />
     <ProjectReference Include="..\AcDream.Plugin.Abstractions\AcDream.Plugin.Abstractions.csproj" />
   </ItemGroup>
 </Project>

tests/AcDream.App.Tests/Platform/GraphicalLegacyConfigurationMigratorTests.cs

@@ -1,5 +1,5 @@
 using AcDream.App.Platform;
-using AcDream.Runtime.Platform;
+using AcDream.Platform;
 
 namespace AcDream.App.Tests.Platform;
 

tests/AcDream.Platform.Tests/AcDream.Platform.Tests.csproj

@@ -0,0 +1,22 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <LangVersion>latest</LangVersion>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\AcDream.Platform\AcDream.Platform.csproj" />
+  </ItemGroup>
+</Project>

tests/AcDream.Platform.Tests/ApplicationPathSetTests.cs

@@ -1,6 +1,6 @@
-using AcDream.Runtime.Platform;
+using AcDream.Platform;
 
-namespace AcDream.Runtime.Tests.Platform;
+namespace AcDream.Platform.Tests;
 
 public sealed class ApplicationPathSetTests
 {

tests/AcDream.Runtime.Tests/RuntimeDependencyBoundaryTests.cs

@@ -72,6 +72,11 @@ public sealed class RuntimeDependencyBoundaryTests
                 "AcDream.Content",
                 "AcDream.Core",
                 "AcDream.Core.Net",
+                // Campaign LA LA0: AcDream.Platform is the new BCL-only
+                // ApplicationPathSet home; Runtime re-exports it
+                // transitively so App/Headless keep reaching it without a
+                // direct reference.
+                "AcDream.Platform",
                 "AcDream.Plugin.Abstractions",
             }
             .Select(projectName => Path.Combine(