M4: Linux + Windows CI build pipeline (.github/workflows/build.yml)

The literal thing nothing on this dev machine can verify: whether the
build actually runs on Windows at all. Solved by having real CI do it —
ubuntu-latest and windows-latest both build the native Box3D/miniaudio
shims from source via CMake, build and test the full .NET solution, stage
a shippable configuration (engine.windowing/assets/render/input/physics/
audio + physics-demo-game — engine.editor deliberately excluded, that's
M4's actual "done when"), and then run it headless against samples/
PhysicsDemo for 200 frames, asserting DemoBox settled at y≈1.0 in the
resulting dump. Real physics, real scene load, real plugin loading,
proven on both platforms, not just built.

Required restructuring the native Content items into per-OS ItemGroups
(native/linux-x64/ vs native/win-x64/, selected via
$([MSBuild]::IsOSPlatform(...))): linux-x64's .so is committed (built and
verified here); win-x64's .dll is never committed — nothing here can
build or run one to verify — and only ever exists as something the
Windows job produces fresh, in-place, right before `dotnet build`.

Caught one real bug dry-running this exact staging locally before
trusting it to a workflow run: PluginHost.Load calls Assembly.
LoadFromAssemblyPath, which throws on a relative path — the verification
step's `--plugins ../../plugins` failed immediately with "is not an
absolute path." Every manual verification earlier this session happened
to always pass an absolute --plugins path, which is exactly why this
never surfaced before. Fixed by resolving to an absolute path before
invoking Engine.Host.

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

plugins/engine.physics/Engine.Physics/Engine.Physics.csproj

@@ -7,22 +7,35 @@
     <EnableDynamicLoading>true</EnableDynamicLoading>
   </PropertyGroup>
 
-  <ItemGroup>
-    <!-- The compiled native shim (Box3D folded in) — see
-         native/physics-native/. Copied straight into this plugin's own
-         output directory (not a runtimes/<rid>/native/ NuGet-style path):
-         DllImport's default unmanaged-library probing already checks the
-         calling assembly's own directory, and PluginLoadContext.
-         LoadUnmanagedDll falls back to that default probing whenever its
-         AssemblyDependencyResolver doesn't recognize the name (returning
-         IntPtr.Zero, not failing outright) — confirmed empirically, see
-         PhysicsWorldTests. -->
-    <Content Include="native/liblingua_physics.so">
+  <!-- The compiled native shim (Box3D folded in) — see
+       native/physics-native/. Copied straight into this plugin's own
+       output directory (not a runtimes/<rid>/native/ NuGet-style path):
+       DllImport's default unmanaged-library probing already checks the
+       calling assembly's own directory, and PluginLoadContext.
+       LoadUnmanagedDll falls back to that default probing whenever its
+       AssemblyDependencyResolver doesn't recognize the name (returning
+       IntPtr.Zero, not failing outright) — confirmed empirically, see
+       PhysicsWorldTests.
+
+       Picked by the BUILDING machine's OS, not cross-compiled: linux-x64's
+       .so is committed (built and verified here); win-x64's .dll is never
+       committed — nothing here can build or run one to verify — and is
+       instead produced fresh by .github/workflows/build.yml's Windows job
+       right before `dotnet build` runs, landing at exactly this path. -->
+  <ItemGroup Condition="$([MSBuild]::IsOSPlatform('Linux'))">
+    <Content Include="native/linux-x64/liblingua_physics.so">
       <Link>liblingua_physics.so</Link>
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
   </ItemGroup>
 
+  <ItemGroup Condition="$([MSBuild]::IsOSPlatform('Windows'))">
+    <Content Include="native/win-x64/lingua_physics.dll">
+      <Link>lingua_physics.dll</Link>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\..\..\src\Engine.Kernel\Engine.Kernel.csproj" />
     <ProjectReference Include="..\Engine.Physics.Contracts\Engine.Physics.Contracts.csproj" />