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_state.h

@@ -15,6 +15,13 @@ enum class Scene {
 
 constexpr int MENU_ITEM_COUNT = 3;
 
+enum class GameOverReason : uint8_t {
+    None = 0,
+    SelfCollision,
+    BarrierCollision,
+    RottenAppleDepleted,
+};
+
 enum class BarrierPhase : uint8_t {
     Inactive, // Slot free
     Warning,  // Flashing announcement, harmless
@@ -47,6 +54,7 @@ struct GameState {
     volatile uint16_t applesEaten = 0;
 
     bool gameOver = false;
+    GameOverReason gameOverReason = GameOverReason::None;
     bool paused = false;
     bool hardMode = false;