feat: add temporary hazardous barriers in hard mode

include/game_state.h

@@ -6,6 +6,8 @@
 #include <vector>
 #include <nds/ndstypes.h>
 
+#include "consts.h"
+
 enum class Scene {
     Menu,
     Playing,
@@ -13,6 +15,23 @@ enum class Scene {
 
 constexpr int MENU_ITEM_COUNT = 3;
 
+enum class BarrierPhase : uint8_t {
+    Inactive, // Slot free
+    Warning,  // Flashing announcement, harmless
+    Active,   // Solid and fatal to the snake head
+};
+
+// Temporary hard-mode wall: a straight line of `length` cells starting at (x, y).
+// Lifetime is tracked in VBlank frames (see tick_barriers_frame()).
+struct Barrier {
+    BarrierPhase phase = BarrierPhase::Inactive;
+    int8_t x = 0;
+    int8_t y = 0;
+    bool vertical = false;
+    uint8_t length = 0;
+    uint16_t framesLeft = 0; // Frames remaining in the current phase
+};
+
 struct GameState {
     volatile uint16_t ticks = 0;
     volatile uint16_t snakelength = 0;
@@ -29,6 +48,11 @@ struct GameState {
 
     uint16_t curseTicksRemaining = 0;
 
+    // Fixed-size (no heap) because barriers are touched from both the VBlank
+    // and Timer 0 IRQs. Those IRQs don't nest, so no further locking is needed.
+    std::array<Barrier, BARRIER_MAX_COUNT> barriers{};
+    volatile uint16_t barrierSpawnCooldown = 0; // Frames until the next spawn attempt
+
     std::deque<std::array<int, 3>> snake;
     std::vector<std::array<int, 2>> apples;
     std::vector<std::array<int, 2>> bad_apples;