Add DEF_PRIMITIVE(list_removeAt) to src/primitives.c that removes an element at a given index from an ObjList, shifts remaining elements up, shrinks capacity when excess is detected via LIST_GROW_FACTOR, and returns the removed Value. Register the primitive as "removeAt " on vm->listClass. Create test/list/remove_at.wren covering removal at positive indices 0,1,2, negative indices -1,-2,-3, out-of-bounds cases returning null, and verification that the removed value is returned.
