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>

source/game_logic.cpp

@@ -47,6 +47,7 @@ bool check_collision_self(GameState &state) {
   for (auto it = state.snake.begin() + 1; it != state.snake.end(); ++it) {
     if ((*it)[0] == headpos[0] && (*it)[1] == headpos[1]) {
       state.gameOver = true;
+      state.gameOverReason = GameOverReason::SelfCollision;
       return true;
     }
   }
@@ -91,6 +92,7 @@ bool check_collision_apples_rotten(GameState &state) {
   state.bad_apples.erase(it);
   if (state.snakelength < 2) {
     state.gameOver = true;
+    state.gameOverReason = GameOverReason::RottenAppleDepleted;
   }
   return true;
 }
@@ -471,6 +473,7 @@ bool check_collision_barriers(GameState &state) {
   const auto &head = state.snake.front();
   if (is_barrier_cell(state, head[0], head[1], false)) {
     state.gameOver = true;
+    state.gameOverReason = GameOverReason::BarrierCollision;
     return true;
   }
   return false;