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>

README.md

@@ -42,8 +42,9 @@
 | **Paused** | **B** | Return to Main Menu |
 | **Paused** | **X** | Toggle background music ON/OFF |
 | **Paused** | **Y** | Toggle Normal / Hard mode and restart |
-| **Game Over** | **START** | Restart game |
+| **Game Over** | **A** or **START** | Restart game |
 | **Game Over** | **B** | Return to Main Menu |
+| **Game Over** | **R Trigger** | Swap top and bottom screens (`lcdSwap`) |
 
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