perf: implement __slots__ in Node class to optimize memory

- Add __slots__ to prevent per-instance dictionary overhead
- Optimize space complexity for large-scale list usage

Author: Iswanna

Sprint-2/implement_linked_list/linked_list.py

@@ -3,6 +3,8 @@ class Node:
     Represents a single person in line.
     Stores a value and links to the people ahead and behind.
     """
+    __slots__ = ['value', 'next', 'previous']
+    
     def __init__(self, value):
         self.value = value
         self.next = None      # The person ahead