- Extract scenes_cube/scenes_sphere (and convenience 'scenes' interface) to scenes/CMakeLists.txt.
- Main CMakeLists.txt now much cleaner (single add_subdirectory(scenes)).
- Updated help text in cuber.cpp to clearly separate Flags vs Keys (less confusing 'S' section).
- No behavior change; depth/viewport/state/post-processing fixes from earlier remain.
Followed project conventions and AGENTS.md commit rules.
