phase(N.5) Task 2: parallel Texture2DArray upload path in TextureCache

Adds UploadRgba8AsLayer1Array — uploads pixel data as a 1-layer
Texture2DArray. Existing UploadRgba8 (Texture2D) untouched, so legacy
callers (StaticMeshRenderer, InstancedMeshRenderer, ParticleRenderer,
WbDrawDispatcher's pre-rewrite path) keep working unchanged.

Required for Task 3's Bindless* methods which need the Texture2DArray
target so the WB modern shader can sample via sampler2DArray. Same
surface may be uploaded both ways during the N.5/N.6 transition;
doubling is bounded and acceptable. After N.6 retires legacy
renderers entirely, the legacy UploadRgba8 becomes unused and is
deleted.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/TextureCache.cs

@@ -279,6 +279,38 @@ public sealed unsafe class TextureCache : Wb.ITextureCachePerInstance, IDisposab
         return tex;
     }
 
+    /// <summary>
+    /// Variant of <see cref="UploadRgba8"/> that uploads pixel data as a 1-layer
+    /// Texture2DArray. Required by the WB modern rendering path which samples via
+    /// sampler2DArray in its bindless shader. Pixel data is identical.
+    /// </summary>
+    private uint UploadRgba8AsLayer1Array(DecodedTexture decoded)
+    {
+        uint tex = _gl.GenTexture();
+        _gl.BindTexture(TextureTarget.Texture2DArray, tex);
+
+        fixed (byte* p = decoded.Rgba8)
+            _gl.TexImage3D(
+                TextureTarget.Texture2DArray,
+                0,
+                InternalFormat.Rgba8,
+                (uint)decoded.Width,
+                (uint)decoded.Height,
+                1,
+                0,
+                PixelFormat.Rgba,
+                PixelType.UnsignedByte,
+                p);
+
+        _gl.TexParameter(TextureTarget.Texture2DArray, TextureParameterName.TextureMinFilter, (int)TextureMinFilter.Linear);
+        _gl.TexParameter(TextureTarget.Texture2DArray, TextureParameterName.TextureMagFilter, (int)TextureMagFilter.Linear);
+        _gl.TexParameter(TextureTarget.Texture2DArray, TextureParameterName.TextureWrapS,     (int)TextureWrapMode.Repeat);
+        _gl.TexParameter(TextureTarget.Texture2DArray, TextureParameterName.TextureWrapT,     (int)TextureWrapMode.Repeat);
+
+        _gl.BindTexture(TextureTarget.Texture2DArray, 0);
+        return tex;
+    }
+
     public void Dispose()
     {
         foreach (var h in _handlesBySurfaceId.Values)