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

AGENTS.md

@@ -39,6 +39,8 @@ The project is split into static libraries:
 - **`cbt_scene`** — Base scene class
 - **`scenes_cube`** — Spinning cube scene implementation
 - **`scenes_sphere`** — Cube-to-sphere mapped mesh with diffuse lighting
+- **`scenes_cornell_box`** — Classic Cornell Box with Lambert shading and
+  a ceiling area light
 
 ### CMake Module Path
 
@@ -89,6 +91,7 @@ to the custom scripts instead of system-installed packages.
 - Use conventional commit prefixes (`feat:`, `fix:`, `docs:`, `chore:`,
   etc.)
 - Separate subject from body with a blank line
+- Do **not** add a `Co-Authored-By` trailer or any agent/AI attribution
 
 Example:
 
@@ -140,6 +143,8 @@ cuber/
     cube.cpp          # Cube scene implementation
     sphere.hpp        # Cube-to-sphere mapped mesh
     sphere.cpp        # Sphere scene implementation
+    cornell_box.hpp   # Cornell Box scene
+    cornell_box.cpp   # Cornell Box scene implementation
   deps/               # Custom Find*.cmake scripts
     Findfmt.cmake     # fmt library
 ```