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/CortexEngine.App/Program.cs

@@ -1,8 +1,10 @@
 using System;
+using System.IO;
 using System.Numerics;
 using Engine.Core;
 using Engine.Core.Components;
 using Engine.Graphics;
+using Engine.Graphics.Loaders;
 using Flecs.NET.Core;
 
 namespace CortexEngine.App;
@@ -11,20 +13,27 @@ class Program
 {
     static void Main(string[] args)
     {
-        Console.WriteLine("Cortex Engine Step 3 — Starting up...");
+        Console.WriteLine("Cortex Engine Step 4 — Starting up...");
 
         try
         {
             using var world = World.Create();
-            using var window = new Sdl3Window("Cortex Engine — Step 3", 1280, 720);
+            using var window = new Sdl3Window("Cortex Engine — Step 4", 1280, 720);
             var timing = new Timing();
             var input = new InputMapping();
             using var vulkan = new VulkanContext(window, enableValidation: false);
             using var swapchain = new Swapchain(vulkan);
-            using var renderer = new TriangleRenderer(vulkan, swapchain);
+            using var renderer = new MeshRenderer(vulkan, swapchain);
 
-            var triangle = world.Entity("Triangle")
-                .Set(new Transform(new Vector3(0.0f, 0.0f, 0.0f)));
+            var modelPath = FindModelPath(args);
+            var mesh = modelPath.EndsWith(".gltf", StringComparison.OrdinalIgnoreCase)
+                || modelPath.EndsWith(".glb", StringComparison.OrdinalIgnoreCase)
+                    ? GltfLoader.Load(modelPath, new Vector3(0.7f, 0.6f, 0.5f))
+                    : ObjLoader.Load(modelPath, new Vector3(0.7f, 0.6f, 0.5f));
+
+            var model = world.Entity("Model")
+                .Set(new Transform(new Vector3(0.0f, 0.0f, 0.0f), Quaternion.Identity, new Vector3(0.5f)))
+                .Set(mesh);
 
             var frames = 0;
             var lastFpsTime = 0.0;
@@ -46,14 +55,10 @@ class Program
                     swapchain.Recreate(lastWidth, lastHeight);
                 }
 
-                // Animate the entity transform.
-                ref var transform = ref triangle.Ensure<Transform>();
-                transform.Position = new Vector3(
-                    MathF.Sin((float)timing.TotalTime) * 0.5f,
-                    0.0f,
-                    0.0f);
-                transform.Rotation = Quaternion.CreateFromAxisAngle(Vector3.UnitZ, (float)timing.TotalTime);
-                transform.Scale = Vector3.One * (1.0f + MathF.Sin((float)timing.TotalTime * 2.0f) * 0.2f);
+                // Slowly rotate the model so we can see it in 3D.
+                ref var transform = ref model.Ensure<Transform>();
+                transform.Rotation = Quaternion.CreateFromAxisAngle(Vector3.UnitY, (float)timing.TotalTime * 0.5f)
+                                   * Quaternion.CreateFromAxisAngle(Vector3.UnitX, (float)timing.TotalTime * 0.25f);
 
                 renderer.RenderWorld(world);
 
@@ -74,4 +79,25 @@ class Program
             Environment.Exit(1);
         }
     }
+
+    private static string FindModelPath(string[] args)
+    {
+        if (args.Length > 0 && File.Exists(args[0]))
+            return args[0];
+
+        var candidates = new[]
+        {
+            "Content/cube.obj",
+            "Models/cube.obj",
+            "cube.obj"
+        };
+
+        foreach (var candidate in candidates)
+        {
+            if (File.Exists(candidate))
+                return candidate;
+        }
+
+        throw new FileNotFoundException("No model file found. Pass a .obj/.gltf/.glb path as argument or place Content/cube.obj next to the executable.");
+    }
 }