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.Graphics.Vulkan/Shaders/vertex.vert

@@ -0,0 +1,48 @@
+#version 450
+
+layout(location = 0) in vec3 inPosition;
+layout(location = 1) in vec3 inColor;
+layout(location = 2) in vec3 inNormal;
+
+layout(location = 0) out vec3 fragColor;
+layout(location = 1) out vec3 fragNormal;
+layout(location = 2) out vec3 fragWorldPos;
+layout(location = 3) out vec2 fragUv;
+
+struct Light
+{
+    vec3 direction;
+    float intensity;
+    vec3 color;
+    float _pad;
+};
+
+layout(set = 0, binding = 0) uniform FrameConstants
+{
+    vec3 cameraPosition;
+    uint lightCount;
+    vec3 ambientColor;
+    float _pad;
+    Light lights[4];
+} frame;
+
+layout(push_constant) uniform PushConstants
+{
+    mat4 mvp;
+    vec3 materialAlbedo;
+    float materialRoughness;
+    float materialMetallic;
+    uint useTexture;
+    uint textureIndex;
+    uint _pad0;
+    uint _pad1;
+} push;
+
+void main()
+{
+    gl_Position = push.mvp * vec4(inPosition, 1.0);
+    fragColor = inColor;
+    fragNormal = inNormal;
+    fragWorldPos = inPosition;
+    fragUv = inPosition.xz * 0.5 + 0.5;
+}