feat: PBR point light — Unity-style attenuation, warm glow

- UBO expanded from 64 to 128 bytes: vp(64) + lightPos(16) + lightColor(16)
- Vertex shader: passes point light UBO data through
- Fragment shader: refactored calcLight() function, shared by directional + point
  - Point light: Unity-style attenuation pow(1 - dist/range, 2)
  - Directional light reduced intensity (0.4) to balance with point light
  - Point light: position (0,8,0), warm color (1,0.9,0.7), intensity 15, range 25
- Renderer: reads Light component from ECS, packs into UBO with Marshal.StructureToPtr
- Scene: MainLight entity with Light.Point at (0,8,0)
- 0 C# struct changes — pure UBO layout + shader upgrade

src/CortexEngine.App/Program.cs

@@ -264,6 +264,10 @@ class Program
             .Set(new Transform(Vector3.Zero, Quaternion.Identity, Vector3.One))
             .Set(grid);
 
+        world.Entity("MainLight")
+            .Set(new Transform(new Vector3(0, 8, 0), Quaternion.Identity, Vector3.One))
+            .Set(Light.Point(new Vector3(0, 8, 0), new Vector3(1.0f, 0.9f, 0.7f), intensity: 15.0f, range: 25.0f));
+
         var entityCount = 0;
         world.Each((Entity e, ref Transform _) => entityCount++);
         Console.WriteLine($"[Scene] {entityCount} entities: torus knot + 4 dynamic cubes + 3 dynamic spheres + static floor + grid");