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.

tests/Engine.Tests/MeshAndLightTests.cs

@@ -25,7 +25,7 @@ public class MeshAndLightTests
     [Fact]
     public void Light_Direction_Is_Normalized()
     {
-        var light = new Light(new Vector3(0, 2, 0), Vector3.One, 1.0f);
+        var light = Light.Directional(new Vector3(0, 2, 0), Vector3.One, 1.0f);
 
         Assert.Equal(1f, light.Direction.Length(), 0.001f);
     }
@@ -33,7 +33,7 @@ public class MeshAndLightTests
     [Fact]
     public void Light_With_Zero_Direction_Defaults_To_UnitY()
     {
-        var light = new Light(Vector3.Zero, Vector3.One, 1.0f);
+        var light = Light.Directional(Vector3.Zero, Vector3.One, 1.0f);
 
         Assert.Equal(Vector3.UnitY, light.Direction);
     }