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_render.cpp

@@ -153,38 +153,62 @@ void draw_apples_cursed(const GameState &state, const RenderAssets &assets,
 
 } // namespace
 
+void draw_game_over_screen(const GameState &state) {
+  iprintf("\x1b[1;2H============================");
+  iprintf("\x1b[2;2H       * GAME OVER *        ");
+  iprintf("\x1b[3;2H============================");
+
+  iprintf("\x1b[5;4HFinal Score:   %5i", state.score);
+  iprintf("\x1b[6;4HApples Eaten:  %5i", state.applesEaten);
+  iprintf("\x1b[7;4HSnake Length:  %5i", state.snakelength);
+  iprintf("\x1b[8;4HDifficulty:    %s", state.hardMode ? "Hard  " : "Normal");
+
+  iprintf("\x1b[10;4HCause of Defeat:");
+  const char *cause = "Collision";
+  switch (state.gameOverReason) {
+  case GameOverReason::SelfCollision:
+    cause = "Bite Collision (Hit Body)";
+    break;
+  case GameOverReason::BarrierCollision:
+    cause = "Energy Barrier Impact";
+    break;
+  case GameOverReason::RottenAppleDepleted:
+    cause = "Depleted by Rotten Apple";
+    break;
+  default:
+    cause = "Collision";
+    break;
+  }
+  iprintf("\x1b[11;4H  %s", cause);
+
+  iprintf("\x1b[14;2H----------------------------");
+  iprintf("\x1b[16;4HA / START : Play Again");
+  iprintf("\x1b[17;4HB         : Main Menu");
+  iprintf("\x1b[18;4HR         : Swap Screens");
+  iprintf("\x1b[20;2H============================");
+}
+
 void invalidate_hud(RenderAssets &assets) { assets.hudCache.dirty = true; }
 
 void draw_game(const GameState &state, const RenderAssets &assets) {
   // OAM slots below BARRIER_OAM_SLOTS are owned by draw_barriers() (VBlank)
   uint16_t oamId = BARRIER_OAM_SLOTS;
 
-  if (!state.gameOver) {
-    draw_apples(state, assets, oamId);
-    draw_apples_rotten(state, assets, oamId);
-    draw_apples_cursed(state, assets, oamId);
-    draw_snake(state, assets, oamId);
+  draw_apples(state, assets, oamId);
+  draw_apples_rotten(state, assets, oamId);
+  draw_apples_cursed(state, assets, oamId);
+  draw_snake(state, assets, oamId);
 
-    // Hide all unused sprites up to hardware limit (128) to eliminate trailing
-    // ghost sprites
-    if (oamId < 128) {
-      oamClear(&oamSub, oamId, 128 - oamId);
-    }
-    return;
+  // Hide all unused sprites up to hardware limit (128) to eliminate trailing
+  // ghost sprites
+  if (oamId < 128) {
+    oamClear(&oamSub, oamId, 128 - oamId);
   }
-
-  // On Game Over, hide all sprites so none are left dangling
-  oamClear(&oamSub, 0, 128);
-
-  iprintf("\x1b[11;12HGAME OVER!");
-  consoleSelect(assets.topScreen);
-  iprintf("\x1b[10;10H  Score: %i    ", state.score);
-  iprintf("\x1b[11;12HGAME OVER!");
 }
 
 void draw_barriers(const GameState &state, const RenderAssets &assets,
                    uint8_t frame) {
-  if (state.scene != Scene::Playing || state.gameOver || !state.hardMode) {
+  if (state.scene != Scene::Playing || !state.hardMode) {
     oamClear(&oamSub, 0, BARRIER_OAM_SLOTS);
     return;
   }
@@ -257,6 +281,12 @@ void draw_barriers(const GameState &state, const RenderAssets &assets,
 
 void draw_stats(const GameState &state, RenderAssets &assets, bool internals,
                 bool musicEnabled, uint8_t frame) {
+  if (state.gameOver) {
+    return;
+  }
+
+  HudCache &cache = assets.hudCache;
+
   if (internals) {
     iprintf("\x1b[0;0HFrame = %i\nTick = %i\ndir = %i", frame, state.ticks,
             state.snakeDirection);
@@ -279,7 +309,6 @@ void draw_stats(const GameState &state, RenderAssets &assets, bool internals,
     }
   }
 
-  HudCache &cache = assets.hudCache;
   const bool cursed = (state.curseTicksRemaining > 0);
 
   const bool needs_score_update = cache.dirty || (cache.score != state.score);
@@ -305,9 +334,7 @@ void draw_stats(const GameState &state, RenderAssets &assets, bool internals,
 
   if (needs_pause_update) {
     if (!state.paused) {
-      if (state.gameOver) {
-        iprintf("\x1b[23;0HB = Main Menu");
-      } else if (cache.paused) {
+      if (cache.paused) {
         // Clear any leftover pause strings if previously paused
         iprintf("\x1b[21;0H             ");
         iprintf("\x1b[22;0H                           ");