feat: add Enumerate method to RouteTable for efficient route enumeration

- Provide a lightweight alternative to `GetAll` by introducing `Enumerate` for on-demand route enumeration without allocating a snapshot list.

Author: ruhex

BGPLite.Routing/RouteTable.cs

@@ -26,6 +26,9 @@ public bool Remove(uint prefix, byte length) =>
     public IReadOnlyList<Route> GetAll() =>
         _routes.Values.ToList();
 
+    /// <summary>Enumerates current routes without materializing a snapshot list (one allocation fewer than GetAll).</summary>
+    public IEnumerable<Route> Enumerate() => _routes.Values;
+
     public void Clear() =>
         _routes.Clear();
 }