Add MrGameEng.Tilemaps: code-built tile grids rendered through the batcher

TileSet maps ids to texture regions with tints (0 = empty), TileGrid is a
bounds-checked dense ushort grid, and the Tilemap component places a grid
in the world per render layer. UseTilemaps() inserts TilemapRenderSystem
before the flush; it submits only the camera-visible cell range
(TilemapMath.VisibleCells), so frame cost scales with the screen, not the
grid. Demonstrated in the sample as a checkerboard floor; Tiled loading
stays on the roadmap on top of this API.

tests/MrGameEng.Tilemaps.Tests/MrGameEng.Tilemaps.Tests.csproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <OutputType>Exe</OutputType>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="xunit.v3" />
+    <PackageReference Include="xunit.runner.visualstudio" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\MrGameEng.Tilemaps\MrGameEng.Tilemaps.csproj" />
+  </ItemGroup>
+
+</Project>