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.

src/MrGameEng.Tilemaps/Tilemap.cs

@@ -0,0 +1,47 @@
+using Friflo.Engine.ECS;
+using Microsoft.Xna.Framework;
+using MrGameEng.Graphics;
+
+namespace MrGameEng.Tilemaps;
+
+/// <summary>
+/// Tilemap component: a <see cref="TileGrid"/> of ids drawn with a <see cref="TileSet"/>.
+/// Cell (0,0) sits at <see cref="Origin"/>, cells are <see cref="TileSize"/> world units square.
+/// Only the cells visible through the camera are submitted each frame, so grids can be large.
+/// Create via the constructor — the struct default has no grid and draws nothing.
+/// </summary>
+public struct Tilemap : IComponent
+{
+    /// <summary>Tile definitions; ids in the grid index into it.</summary>
+    public TileSet? TileSet;
+
+    /// <summary>The cells. Mutating ids takes effect next frame.</summary>
+    public TileGrid? Grid;
+
+    /// <summary>World position of the top-left corner of cell (0,0).</summary>
+    public Vector2 Origin;
+
+    /// <summary>Cell size in world units.</summary>
+    public float TileSize;
+
+    /// <summary>Render layer of the whole map.</summary>
+    public LayerId Layer;
+
+    /// <summary>Draw order within the layer (smaller = drawn first / behind).</summary>
+    public float Depth;
+
+    /// <summary>Tint multiplied into every tile on top of its <see cref="TileDef.Color"/>.</summary>
+    public Color Color;
+
+    /// <summary>Creates a tilemap at world origin.</summary>
+    public Tilemap(TileGrid grid, TileSet tileSet, float tileSize, LayerId layer = default)
+    {
+        Grid = grid;
+        TileSet = tileSet;
+        TileSize = tileSize;
+        Layer = layer;
+        Origin = Vector2.Zero;
+        Depth = 0f;
+        Color = Color.White;
+    }
+}