feat: add custom background rendering and speed scaling features for snake gameplay

include/game_bg.h

@@ -0,0 +1,34 @@
+#ifndef GAME_BG_H
+#define GAME_BG_H
+
+#include <nds.h>
+#include "game_state.h"
+
+struct BackgroundContext {
+    int bg_main_id = -1;
+    int bg_sub_id = -1;
+
+    // Fixed-point (8.8) scroll accumulators for smooth 60fps drift
+    uint16_t scroll_main_x_fp = 0;
+    uint16_t scroll_main_y_fp = 0;
+    uint16_t scroll_sub_x_fp = 0;
+    uint16_t scroll_sub_y_fp = 0;
+
+    // Animation frame counter & timers
+    uint8_t anim_phase = 0;
+    uint16_t anim_phase_fp = 0; // Fixed-point (8.8) phase accumulator
+    uint8_t pulse_timer = 0; // Triggered when an apple is eaten
+
+    // Cached state to optimize updates
+    bool hard_mode = false;
+    bool is_cursed = false;
+    bool game_over = false;
+    Scene scene = Scene::Menu;
+};
+
+void bg_setup(BackgroundContext &bg_ctx, int top_console_bg_id, int bottom_console_bg_id);
+void bg_update(BackgroundContext &bg_ctx, const GameState &state, uint8_t frame);
+void bg_trigger_pulse(BackgroundContext &bg_ctx);
+void bg_set_scene(BackgroundContext &bg_ctx, Scene scene, bool hard_mode);
+
+#endif // GAME_BG_H