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

include/consts.h

@@ -3,11 +3,18 @@
 #endif
 
 const int START_LENGTH = 4;
-const int START_LENGTH_HARD =2;
+const int START_LENGTH_HARD = 2;
 const int APPLE_SCORE = 10;
 const int CURSE_DURATION_TICKS = 40;
 
-//Screen size
+// Speed scaling configuration (gradual acceleration based on consumed apples)
+const int SPEED_BASE_NORMAL_MHZ = 5000;  // 5.0 Hz (~200 ms per step)
+const int SPEED_BASE_HARD_MHZ   = 9000;  // 9.0 Hz (~111 ms per step)
+const int SPEED_MAX_NORMAL_MHZ  = 13000; // 13.0 Hz (~77 ms per step)
+const int SPEED_MAX_HARD_MHZ    = 17000; // 17.0 Hz (~59 ms per step)
+const int SPEED_APPLES_CAP      = 40;    // Apples consumed to reach max intensity & speed
+
+// Screen size
 const int X_MAX = 31;
 const int Y_MAX = 23;