fix: decouple VBlank ISR and main loop to prevent IRQ stack overflow

- Make handle_vblank strictly increment frame counter (frame++)
- Move heavy frame logic (background updates, OAM updates, palette animations, and console stats) to update_frame() on the main thread synchronized with swiWaitForVBlank()
- Eliminate 256-byte ARM9 IRQ stack (sp_irq) overflow caused by bg_update and iprintf that was corrupting DTCM stack and causing data aborts
- Preserve all playfield entities on game over with top-screen defeat summary and smooth background deceleration

Co-authored-by: Antigravity <antigravity@google.com>

include/game_bg.h

@@ -23,6 +23,7 @@ struct BackgroundContext {
     bool hard_mode = false;
     bool is_cursed = false;
     bool game_over = false;
+    uint8_t game_over_fade = 0;
     Scene scene = Scene::Menu;
 };