feat: Step 4 load .obj and glTF models into ECS

- Add Vertex struct and Mesh ECS component (vertices + indices).
- Add Vulkan IndexBuffer for indexed draws.
- Add MeshRenderer that draws Mesh + Transform entities with vkCmdDrawIndexed.
- Add minimal .obj loader (positions, faces) and glTF/glTF-binary loader via SharpGLTF.Core.
- Rewrite shaders to 3D position + color; recompile to SPIR-V.
- Update VulkanPipeline for new vertex format.
- Ship a sample Content/cube.obj and wire Program.cs to load it.
- Remove TriangleRenderer (replaced by MeshRenderer).

src/Engine.Core/Vertex.cs

@@ -0,0 +1,19 @@
+using System.Numerics;
+
+namespace Engine.Core;
+
+/// <summary>
+/// A simple 3D vertex with position and color.
+/// Layout matches the Vulkan vertex input description.
+/// </summary>
+public struct Vertex
+{
+    public Vector3 Position;
+    public Vector3 Color;
+
+    public Vertex(Vector3 position, Vector3 color)
+    {
+        Position = position;
+        Color = color;
+    }
+}