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

src/Engine.AI/Stdio/McpStdioServer.cs

@@ -55,7 +55,7 @@ public sealed class McpStdioServer
                 "List all named entities in the ECS world.",
                 new JsonSchemaBuilder().Build()),
             ["CaptureScreenshot"] = new(
-                "Capture a screenshot of the current rendered frame and save it to disk.",
+                "Capture a screenshot of the current rendered frame and save it to disk. (No graphics context in stdio mode; returns requested path.)",
                 new JsonSchemaBuilder()
                     .AddOptionalString("outputPath")
                     .Build()),