Add a new Elite NPC zombie called 'The Leaper' (Stalker) that spawns in towns. He is an agile, ceiling-clinging zombie with max HP of 0.75x a normal zombie and 1.15x runner's speed. He has red glowing eyes. When his health drops to 50%, he retreats and switches to 'Stalker Mode,' attacking from behind or hiding on ceilings. He can cling to ceilings inside buildings and is silent except for heavy breathing when a player is directly underneath. His appearance includes black jeans, a white tank top, and medium-length hair.