Hello: overhaul to match ObjectivelyMVC Hello structure and style

- License header: fix 'MVC framework' -> 'GPU layer'
- Add Scene struct; move GPU resources and angles out of flat AppState
- AppState uses AppState *app pointer style (not app. globals)
- static AppState application (not app)
- Framebuffer initialized with swapchain color format + D16_UNORM
- initScene(AppState *app): uses app->framebuffer->colorFormat/depthFormat
- drawScene(AppState *app, CommandBuffer *cmd): extracted from SDL_AppIterate,
  uses SDL_fmodf, derives aspect ratio from framebuffer->size
- SDL_AppIterate: resize FBO, drawScene, blit to swapchain, submit
- Swapchain early-exit guard (cancel + release cmd if no texture)
- SDL_AppQuit: no null checks; waitForIdle then direct release

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jdolan