feat: add Cornell Box scene with orbit camera

- gfx::pipeline gains bind_vec3/bind_float for passing arbitrary
  uniforms to shaders (used for light position/color)
- scene base gains on_mouse_drag virtual hook; cuber.cpp polls
  left-mouse delta each frame and forwards it to the active scene
- cornell_box scene: 5-wall room (red/green/white), two pre-rotated
  white boxes with proportions from the original paper, an unlit
  ceiling light panel, and a point-light Lambert shader
- left-click drag orbits the camera around the scene origin;
  pitch is clamped to ±80° to prevent gimbal flip
- key 3 / --scene cornell_box selects the scene

cbt/gfx.hpp

@@ -50,6 +50,8 @@ public:
     auto valid() const -> bool;
     auto draw(glm::mat4 const& model, glm::mat4 const& view, glm::mat4 const& proj) const -> void;
     auto bind_texture(char const* sampler_name, std::uint32_t texture_id, std::uint32_t unit = 0) const -> void;
+    auto bind_vec3(char const* name, glm::vec3 const& v) const -> void;
+    auto bind_float(char const* name, float v) const -> void;
 
 private:
     struct impl;