feat: Step 3 Flecs.NET ECS integration with animated triangle

- Add Transform component and ECS world in Engine.Core.
- TriangleRenderer.RenderWorld queries entities and applies transforms.
- Animate a single triangle entity in Program.cs.
- Pin ppy.SDL3-CS to 2026.520.0 and add linux-x64 RID for bundled native SDL3.
- Add Flecs.NET.Debug/Release 4.0.4-build.546 per configuration.
- Update SDL3 API usage for the 2026.520.0 binding.
- Update architecture file to Silk.NET.Vulkan and Flecs 4.0.4.

src/CortexEngine.App/CortexEngine.App.csproj

@@ -11,6 +11,7 @@
 
   <PropertyGroup Condition="'$(Configuration)' == 'Debug'">
     <DefineConstants>DEV_MODE</DefineConstants>
+    <RuntimeIdentifier>linux-x64</RuntimeIdentifier>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)' == 'ReleaseAOT'">

src/CortexEngine.App/Program.cs

@@ -1,6 +1,9 @@
 using System;
+using System.Numerics;
 using Engine.Core;
+using Engine.Core.Components;
 using Engine.Graphics;
+using Flecs.NET.Core;
 
 namespace CortexEngine.App;
 
@@ -8,17 +11,21 @@ class Program
 {
     static void Main(string[] args)
     {
-        Console.WriteLine("Cortex Engine Step 2 — Starting up...");
+        Console.WriteLine("Cortex Engine Step 3 — Starting up...");
 
         try
         {
-            using var window = new Sdl3Window("Cortex Engine — Step 2", 1280, 720);
+            using var world = World.Create();
+            using var window = new Sdl3Window("Cortex Engine — Step 3", 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);
 
+            var triangle = world.Entity("Triangle")
+                .Set(new Transform(new Vector3(0.0f, 0.0f, 0.0f)));
+
             var frames = 0;
             var lastFpsTime = 0.0;
             var lastWidth = window.Width;
@@ -39,7 +46,16 @@ class Program
                     swapchain.Recreate(lastWidth, lastHeight);
                 }
 
-                renderer.RenderFrame();
+                // 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);
+
+                renderer.RenderWorld(world);
 
                 frames++;
                 if (timing.TotalTime - lastFpsTime >= 1.0)