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.AI/AiCommandProcessor.cs

@@ -224,7 +224,16 @@ public sealed class AiCommandProcessor
                 {
                     ref var light = ref e.Ensure<Light>();
                     writer.WriteStartObject("Light");
-                    WriteVector3(writer, "direction", light.Direction);
+                    writer.WriteString("type", light.Type.ToString());
+                    if (light.IsPoint)
+                    {
+                        WriteVector3(writer, "position", light.Position);
+                        writer.WriteNumber("range", light.Range);
+                    }
+                    else
+                    {
+                        WriteVector3(writer, "direction", light.Direction);
+                    }
                     WriteVector3(writer, "color", light.Color);
                     writer.WriteNumber("intensity", light.Intensity);
                     writer.WriteEndObject();