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_render.h

@@ -42,6 +42,7 @@ void invalidate_hud(RenderAssets &assets);
 void draw_game(const GameState &state, const RenderAssets &assets);
 void draw_stats(const GameState &state, RenderAssets &assets, bool internals, bool musicEnabled, uint8_t frame);
 void draw_menu(const GameState &state, RenderAssets &assets, bool musicEnabled);
+void draw_game_over_screen(const GameState &state);
 void blink_apples(const GameState &state, u16 *gfx_apple);
 // Called every VBlank: owns OAM slots [0, BARRIER_OAM_SLOTS) and animates barrier palette entries
 void draw_barriers(const GameState &state, const RenderAssets &assets, uint8_t frame);