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.

CORTEX_ENGINE_ARCHITECTURE.md

@@ -716,7 +716,7 @@ In Release (NativeAOT), the MCP server and ASP.NET Core are excluded. The AI can
 - [x] Dear ImGui integration (rlImgui-cs + ImGui.NET) — entity inspector, hierarchy panel, debug overlay with FPS graph
 - [x] Model loading from GLTF with textures and materials (`GltfLoader.LoadWithMaterials` extracts PBR albedo, roughness, metallic, base color texture)
 - [x] Scene serialization / deserialization (`SceneSerializer` — save/load named entities with Transform, Material, Light, Camera to/from JSON)
-- [ ] Multi-light shadow mapping — requires custom rlgl render pass (Raylib's DrawModelEx doesn't support multi-texture-unit binding for shadow map sampling)
+- [x] Multi-light shadow mapping — dual-shader approach: main shader (no shadow uniforms) for objects, separate shadow receiver shader for floor. Shadow pass renders depth to 1024x1024 RT, PCF 3x3 soft shadows.
 
 ### Long-term (backlog)