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/main.cpp

@@ -205,19 +205,33 @@ void setup() {
 }
 void handle_vblank() {
   frame++;
+}
+
+void update_frame() {
   bg_update(bgContext, gameState, frame);
   if (gameState.scene == Scene::Playing) {
     consoleSelect(&topScreen);
-    draw_stats(gameState, renderAssets, internals,
-               audio_is_music_enabled(audioContext), frame);
-    if (!gameState.paused) {
-      tick_barriers_frame(gameState);
+    if (gameState.gameOver) {
+      HudCache &cache = renderAssets.hudCache;
+      if (cache.dirty || (cache.gameOver != gameState.gameOver)) {
+        consoleClear();
+        draw_game_over_screen(gameState);
+        cache.gameOver = gameState.gameOver;
+        cache.dirty = false;
+      }
+    } else {
+      draw_stats(gameState, renderAssets, internals,
+                 audio_is_music_enabled(audioContext), frame);
+      if (!gameState.paused) {
+        tick_barriers_frame(gameState);
+      }
     }
     draw_barriers(gameState, renderAssets, frame);
   }
 
   // Dynamic pulsing radioactive glow for Cursed Apples (smooth 60fps breathing)
-  const uint8_t pulse = (frame >> 2U) & 7U; // 8-step cycle
+  const auto pulse = static_cast<uint8_t>(
+      (static_cast<uint32_t>(frame) >> 2U) & 7U); // 8-step cycle
   const uint8_t wave =
       (pulse < 4U) ? pulse
                    : static_cast<uint8_t>(7U - pulse); // 0, 1, 2, 3, 3, 2, 1, 0
@@ -233,8 +247,9 @@ void handle_vblank() {
 
   // Dynamic festering mold & spore respiration for Rotten Apples (smooth 60fps
   // organic pulse)
-  const uint8_t rot_pulse = ((frame + 3U) >> 2U) &
-                            7U; // 8-step cycle, phase-shifted from cursed apple
+  const auto rot_pulse = static_cast<uint8_t>(
+      (static_cast<uint32_t>(frame + 3U) >> 2U) &
+      7U); // 8-step cycle, phase-shifted from cursed apple
   const uint8_t rot_wave =
       (rot_pulse < 4U)
           ? rot_pulse
@@ -292,6 +307,7 @@ void do_tick() {
       if (!was_game_over && gameOver) {
         audio_stop_music();
         audio_play_fail(audioContext);
+        invalidate_hud(renderAssets);
       }
       // consoleSelect(&bottomScreen);
       draw_game(gameState, renderAssets);
@@ -309,6 +325,7 @@ void init_game() {
   score = 0;
   ticks = 0;
   gameOver = false;
+  gameState.gameOverReason = GameOverReason::None;
   snakeDirection = 1;
   gameState.curseTicksRemaining = 0;
   apples.clear();
@@ -338,10 +355,6 @@ void init_game() {
 }
 
 int main(void) {
-
-  // Set up VBlank handler
-  irqSet(IRQ_VBLANK, handle_vblank);
-
   setup();
   setBackdropColor(RGB15(2, 2, 6));
   setBackdropColorSub(RGB15(2, 2, 6));
@@ -354,7 +367,12 @@ int main(void) {
   inputContext.topScreen = &topScreen;
   inputContext.init_game = init_game;
 
+  // Set up VBlank handler only after all subsystems, assets, and contexts are initialized
+  irqSet(IRQ_VBLANK, handle_vblank);
+
   while (1) {
+    swiWaitForVBlank();
+    update_frame();
     if (!handle_input_frame(inputContext)) {
       break;
     }