refactor: move scenes to dedicated CMakeLists.txt + clean arg parsing help

- 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.

cuber.cpp

@@ -17,18 +17,21 @@
 
 auto main(int argc, char const* argv[]) -> int {
     float max_duration_seconds = 0.0f;
-    std::string_view scene_name = "cube";
+    std::string scene_name = "cube";  // "cube", "sphere", or "2" (for compatibility with keys)
     bool take_screenshot = false;
 
     for (int i = 1; i < argc; ++i) {
         std::string_view arg = argv[i];
         if (arg == "--help" || arg == "-h") {
             fmt::print("Usage: {} [options]\n", argv[0]);
+            fmt::print("Flags:\n");
             fmt::print("  --duration <seconds>  Auto-terminate after N seconds (for testing/CI)\n");
             fmt::print("  --scene <cube|sphere> Select initial scene (default: cube)\n");
             fmt::print("  --screenshot          Render one frame, save screenshot, and exit\n");
-            fmt::print("  S key                 Take screenshot (saved as screenshot.png)\n");
-            fmt::print("  1/2 key               Switch between cube/sphere scene\n");
+            fmt::print("\nKeys (during runtime):\n");
+            fmt::print("  S                     Take screenshot (saved as screenshot.png)\n");
+            fmt::print("  1/2                   Switch between cube/sphere scene\n");
+            fmt::print("  Q                     Quit\n");
             return 0;
         }
         if (arg == "--duration" && i + 1 < argc) {
@@ -36,7 +39,7 @@ auto main(int argc, char const* argv[]) -> int {
             continue;
         }
         if (arg == "--scene" && i + 1 < argc) {
-            scene_name = argv[++i];
+            scene_name = argv[++i];  // std::string to avoid string_view lifetime gotchas with argv
             continue;
         }
         if (arg == "--screenshot") {