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).

Content/cube.obj

@@ -0,0 +1,33 @@
+# Simple unit cube
+v -0.5 -0.5 -0.5
+v  0.5 -0.5 -0.5
+v  0.5  0.5 -0.5
+v -0.5  0.5 -0.5
+v -0.5 -0.5  0.5
+v  0.5 -0.5  0.5
+v  0.5  0.5  0.5
+v -0.5  0.5  0.5
+
+# Back face
+f 1 2 3
+f 1 3 4
+
+# Front face
+f 5 7 6
+f 5 8 7
+
+# Left face
+f 1 5 6
+f 1 6 2
+
+# Right face
+f 3 7 8
+f 3 8 4
+
+# Bottom face
+f 1 4 8
+f 1 8 5
+
+# Top face
+f 2 6 7
+f 2 7 3