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/MeshMathAndProceduralTests.cs

@@ -14,9 +14,10 @@ public class MeshMathTests
             new Vector3(1, 0, 0),
             new Vector3(0, 1, 0));
 
+        // CW winding (typical OBJ) → normal points -Z
         Assert.Equal(0f, n.X, 0.001f);
         Assert.Equal(0f, n.Y, 0.001f);
-        Assert.Equal(1f, n.Z, 0.001f);
+        Assert.Equal(-1f, n.Z, 0.001f);
     }
 
     [Fact]