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/scene.cpp

@@ -10,4 +10,6 @@ auto scene::update(float) -> void {}
 
 auto scene::render(int, int) -> void {}
 
+auto scene::on_mouse_drag(double, double) -> void {}
+
 }