feat(session): live furnidata reload via mergeFurnitureDataFromUrl

Add SessionDataManager.mergeFurnitureDataFromUrl() + FurnitureDataLoader.mergeFromUrl()
to merge a single furnidata chunk (e.g. a custom tier) into the live floor/wall maps at
runtime, returning the added entries so callers can also refresh the RoomContentLoader.
Lets newly added furniture appear without a full client reload.

packages/api/src/nitro/session/ISessionDataManager.ts

@@ -9,6 +9,7 @@ export interface ISessionDataManager
 {
     init(): Promise<void>;
     getAllFurnitureData(): IFurnitureData[];
+    mergeFurnitureDataFromUrl(url: string): Promise<IFurnitureData[]>;
     applyFurnitureDataOverrides(url: string): Promise<void>;
     clearFurnitureDataOverrides(): void;
     getFloorItemData(id: number): IFurnitureData;