feat: render spinning 3D cube with glm

- Add glm dependency for matrix transformations
- Replace triangle with 6-face colored cube
- Add MVP shader uniforms (model, view, projection)
- Enable depth testing for proper 3D rendering
- Spin cube around (1, 0.5, 0.3) axis

style: fix trailing return type for move assignment operators

- buffer/texture/shader/vao: use auto fn() -> T& style
- Document trailing return type convention in AGENTS.md

CMakeLists.txt

@@ -16,6 +16,7 @@ find_package(fmt REQUIRED)
 find_package(glfw3 REQUIRED)
 find_package(glad REQUIRED)
 find_package(asio REQUIRED)
+find_package(glm REQUIRED)
 
 # Target setup
 add_executable(cuber "cuber.cpp"
@@ -30,4 +31,4 @@ target_include_directories(cuber PRIVATE ".")
 target_compile_features(cuber PRIVATE cxx_std_23)
 target_compile_options(cuber PRIVATE ${BASE_OPTIONS})
 target_compile_definitions(cuber PRIVATE ${BASE_DEFINITIONS})
-target_link_libraries(cuber PRIVATE fmt::fmt glfw::glfw glad::glad asio::asio ${BASE_LIBRARIES})
+target_link_libraries(cuber PRIVATE fmt::fmt glfw::glfw glad::glad asio::asio glm::glm ${BASE_LIBRARIES})