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

AGENTS.md

@@ -38,11 +38,12 @@ to the custom scripts instead of system-installed packages.
 ## Coding Conventions
 
 - **Language:** C++23
-- **Trailing return type** for function signatures
+- **Trailing return type** for function signatures (e.g. `auto fn() -> void`)
 - **4-space indentation**
 - **No semicolons after closing braces** for namespaces/classes
 - `auto` for obvious types (e.g. `auto main(...) -> int`)
 - **East const** (e.g. `char const*` not `const char*`)
+- **Trailing return type** for all function definitions, including operators (e.g. `auto operator=(T&&) noexcept -> T&`)
 - `<>` includes only for system headers (std, OS, etc.)
 - `""` includes for third-party dependencies (e.g. `fmt`, `nlohmann/json`)
 - **Naming:** `snake_case` for variables, functions, and classes