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