feat: add sphere scene with fixed cube-to-sphere mapping

Added scenes/sphere.{hpp,cpp} using the cube-to-sphere
approach from nrz.cpp, but with corrected math: vertices
are simply normalized to project onto the unit sphere
(the original used a broken formula with p=50.0 as an
exponent).

The sphere uses indexed geometry with position, normal,
and UV attributes, plus a simple diffuse lighting shader.

Press 1/2 to switch between cube and sphere scenes.
Updated .gitignore to exclude generated PNG screenshots.

scenes/sphere.hpp

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <chrono>
+
+#include "glm/glm.hpp"
+
+#include "cbt/scene.hpp"
+#include "cbt/opengl/buffer.hpp"
+#include "cbt/opengl/shader.hpp"
+#include "cbt/opengl/vao.hpp"
+
+namespace cbt::scenes {
+
+class sphere final : public scene {
+public:
+    sphere();
+    auto init() -> bool override;
+    auto update(float delta_time) -> void override;
+    auto render() -> void override;
+
+private:
+    opengl::shader m_prog;
+    opengl::buffer m_vbo;
+    opengl::buffer m_ebo{opengl::buffer_type::index};
+    opengl::vao m_vao;
+
+    GLint m_loc_proj = -1;
+    GLint m_loc_view = -1;
+    GLint m_loc_model = -1;
+
+    GLsizei m_index_count = 0;
+
+    std::chrono::steady_clock::time_point m_start;
+
+    auto build_mesh() -> void;
+    auto build_shader() -> bool;
+};
+
+}