Add group for loadouts (#36951)

Co-authored-by: ArtisticRoomba <145879011+ArtisticRoomba@users.noreply.github.com>

Content.Client/Lobby/UI/Loadouts/SubLoadoutContainer.xaml.cs

@@ -0,0 +1,21 @@
+using Robust.Client.AutoGenerated;
+using Robust.Client.UserInterface.Controls;
+
+namespace Content.Client.Lobby.UI.Loadouts;
+
+/// <summary>
+/// A simple container used to group additional loadout UI elements 
+/// that are part of the same GroupBy subgroup.
+/// 
+/// - Used when a loadout group contains multiple prototypes.
+/// - The first prototype is shown directly; the remaining ones are placed inside this container.
+/// - Allows toggling visibility of the subgroup via expandable UI (collapsible behavior).
+/// 
+/// Internally inherits from PanelContainer to allow for border/background styling if needed.
+/// Exposes its internal BoxContainer (SubGridContainer) via the <see cref="Grid"/> property for adding children.
+/// </summary>
+[GenerateTypedNameReferences]
+public sealed partial class SubLoadoutContainer : PanelContainer
+{
+    public BoxContainer Grid => SubGridContainer;
+}