feat: modular HAL, Raylib backend, PBR shading, textures, 60 unit tests

- Replace hardcoded SDL3 windowing with IWindow/IInputState/Key abstractions
- Each render backend owns its window (Raylib GLFW, SDL3 for Vulkan)
- Raylib backend: DrawModelEx, custom GLSL shader with Fresnel, ACES
  tonemapping, gamma correction, hemisphere ambient
- Fix backface culling, mesh memory (NativeMemory.Alloc), texture loading
- Camera controllers use backend-agnostic Key enum (inverted yaw/strafe)
- Demo scene: 8 cubes, 7 spheres, torus knot OBJ with checker texture
- Extract ProceduralMesh + MeshMath from Program.cs to Engine.Graphics
- Vulkan backend deferred (compiles, untested, IWindow-compatible)
- 60 unit tests: ObjLoader, camera controllers, AiCommandProcessor,
  RenderBackendFactory, Timing, ProceduralMesh, MeshMath, Transform
- AGENTS.md for opencode integration

AGENTS.md

@@ -0,0 +1,66 @@
+# AGENTS.md — Cortex Engine
+
+## Project Overview
+
+Cortex Engine is a C# (.NET 9) AI-Native 3D game engine. The primary render backend is Raylib-cs (OpenGL). A Vulkan backend exists but is deferred.
+
+## Build Commands
+
+```bash
+# Build (Debug)
+dotnet build CORTEX_ENGINE.sln -c Debug
+
+# Build (Release)
+dotnet build CORTEX_ENGINE.sln -c Release
+
+# Run the engine
+./scripts/run.sh
+
+# Run with test scene + camera tour (headless screenshot capture)
+dotnet run --project src/CortexEngine.App/CortexEngine.App.csproj -c Release -- --test-scene --camera-tour --mcp-port 0
+
+# Run with MCP server
+./scripts/run.sh --mcp-port 5000
+```
+
+## Lint / Typecheck
+
+No separate lint command. `dotnet build` with 0 warnings is the standard. Run `dotnet build CORTEX_ENGINE.sln -c Release` to verify.
+
+## Architecture
+
+- **Engine.Core** — `IWindow`, `IInputState`, `Key` enum, `Sdl3Window`, camera controllers, ECS components (`Transform`, `Mesh`, `Material`, `Light`, `Camera`), `Timing`
+- **Engine.Graphics** — HAL interfaces (`IRenderContext`, `IRenderer`), `RenderBackendFactory`, mesh loaders (`ObjLoader`, `GltfLoader`)
+- **Engine.Graphics.Raylib** — Primary backend. `RaylibWindow` (GLFW), `RaylibInputState`, `RaylibRenderer` with custom GLSL 330 shader (Fresnel, ACES, gamma)
+- **Engine.Graphics.Vulkan** — Deferred backend. Compiles but untested. Uses `Sdl3Window` for Vulkan surface.
+- **Engine.AI** — `AiCommandProcessor` (7 commands), MCP HTTP + stdio servers
+- **CortexEngine.App** — Entry point, main loop, scene setup
+
+## Key Conventions
+
+- Each render backend owns its window (`IWindow`). The app gets the window from `IRenderContext.Window`.
+- Input is backend-agnostic via `IInputState` + `Key` enum. No SDL3 types in app code.
+- Camera controllers use `IInputState`, not `InputMapping` directly.
+- `RenderBackendFactory.Create(name, width, height, validation)` — backends register by name.
+- Custom mesh CPU data uses `NativeMemory.Alloc` (not `Marshal.AllocHGlobal`) to match Raylib's `RL_FREE`.
+- `SetShaderValue` uses `float[]` for vectors, not `Vector3`/`Vector4` (marshaling reliability).
+- Backface culling disabled (`Rlgl.DisableBackfaceCulling`) for mixed-winding meshes.
+
+## Current Roadmap
+
+See `CORTEX_ENGINE_ARCHITECTURE.md` §11 for the full roadmap. Short-term priorities:
+- Unit tests
+- Texture loading verification
+- ImGui integration (medium-term)
+
+## Files Not to Edit
+
+- `CORTEX_ENGINE_ARCHITECTURE.md` — canonical architecture reference, update only when architecture changes
+- `src/Engine.Graphics.Vulkan/Shaders/*.spv` — compiled SPIR-V, regenerate from `.vert`/`.frag` with glslangValidator
+
+## Environment
+
+- .NET 9 SDK at `$HOME/.dotnet`
+- `DOTNET_ROOT` and `PATH` must include `$HOME/.dotnet`
+- Raylib-cs 8.0.0 (Raylib 6.0 native library bundled in NuGet)
+- Display required (X11/Wayland) for Raylib window