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

scenes/cornell_box.hpp

@@ -0,0 +1,32 @@
+#pragma once
+
+#include "glm/glm.hpp"
+
+#include "cbt/scene.hpp"
+#include "cbt/gfx.hpp"
+
+namespace cbt::scenes {
+
+class cornell_box final : public scene {
+public:
+    auto init() -> bool override;
+    auto update(float delta_time) -> void override;
+    auto render(int width, int height) -> void override;
+    auto on_mouse_drag(double dx, double dy) -> void override;
+
+private:
+    gfx::pipeline m_scene_pipeline;
+    gfx::pipeline m_light_pipeline;
+    gfx::pipeline m_post_pipeline;
+    gfx::render_target m_rt{0, 0};
+
+    float m_yaw    = 0.0f;  // degrees, horizontal orbit
+    float m_pitch  = 0.0f;  // degrees, vertical orbit
+    float m_radius = 4.0f;
+
+    auto build_scene_pipeline() -> bool;
+    auto build_light_pipeline() -> bool;
+    auto build_post_pipeline() -> bool;
+};
+
+}