fix: rlgl depth FBO shadows, correct normals, linear gamma, per-channel Fresnel

- Shadow mapping via rlgl: custom depth FBO (2048x2048, 24-bit depth texture)
  instead of color-attachment approach. Proper depth-only render pass.
- Shadow map bound via MaterialMapIndex.Emission (texture unit 1), sampled
  in shadow receiver shader with PCF 3x3 soft shadows.
- Fixed inverted normals: Cross(ac, ab) for CW winding in OBJ files.
- Linear workflow: pow(albedo, 2.2) before lighting, pow(result, 1/2.2) after.
- Per-channel Fresnel: vec3 F0 + (1-F0)*pow(1-HdotV,5) instead of F0.x.
- Single CollectLights call after BeginMode3D.
- Shadow pass after BeginDrawing (inside frame).
- Backface culling disabled globally for mixed-winding meshes.
- Point light support: Light.Point/Directional factory methods, attenuation,
  ImGui inspector with type combo, position/range for point lights.
- Floor rendered for both light types (shadow receiver or main shader).
- 66/66 tests passing.

src/Engine.Graphics/MeshMath.cs

@@ -15,7 +15,8 @@ public static class MeshMath
     {
         var ab = b - a;
         var ac = c - a;
-        var normal = Vector3.Cross(ab, ac);
+        // Cross(ac, ab) instead of Cross(ab, ac) to match CW winding in typical OBJ files
+        var normal = Vector3.Cross(ac, ab);
         if (normal.LengthSquared() > 0.00001f)
             normal = Vector3.Normalize(normal);
         else

src/Engine.Graphics/SceneSerializer.cs

@@ -74,9 +74,12 @@ public static class SceneSerializer
                 var l = e.Get<Light>();
                 entry.Light = new SceneLight
                 {
+                    Type = l.Type,
                     Direction = l.Direction,
+                    Position = l.Position,
                     Color = l.Color,
-                    Intensity = l.Intensity
+                    Intensity = l.Intensity,
+                    Range = l.Range
                 };
             }
 
@@ -144,10 +147,21 @@ public static class SceneSerializer
 
             if (entry.Light != null)
             {
-                entity.Set(new Light(
-                    entry.Light.Direction,
-                    entry.Light.Color,
-                    entry.Light.Intensity));
+                if (entry.Light.Type == LightType.Point)
+                {
+                    entity.Set(Light.Point(
+                        entry.Light.Position,
+                        entry.Light.Color,
+                        entry.Light.Intensity,
+                        entry.Light.Range));
+                }
+                else
+                {
+                    entity.Set(Light.Directional(
+                        entry.Light.Direction,
+                        entry.Light.Color,
+                        entry.Light.Intensity));
+                }
             }
 
             if (entry.Camera != null)
@@ -208,9 +222,12 @@ internal sealed class SceneMaterial
 
 internal sealed class SceneLight
 {
+    public LightType Type { get; set; }
     public Vector3 Direction { get; set; }
+    public Vector3 Position { get; set; }
     public Vector3 Color { get; set; }
     public float Intensity { get; set; }
+    public float Range { get; set; }
 }
 
 internal sealed class SceneCamera