fix(save): restore visited set before setting current room; strengthen null-tolerance test

Reorder apply() so visitedRoomIds is restored before setCurrentRoom() is
called, ensuring the current room is never accidentally stripped from the
visited set by the subsequent clear(). Also adds a post-condition assertion
to applyToleratesMissingCollections that proves null roomItemIds leaves
room item tables empty after apply.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Semesterprojekt/src/main/java/thb/jeanluc/adventure/save/SaveCodec.java

@@ -95,9 +95,9 @@ public final class SaveCodec {
         }
 
         // 3. Player position + visited + gold.
-        p.setCurrentRoom(requireRoom(w, data.currentRoomId()));
         p.getVisitedRoomIds().clear();
         p.getVisitedRoomIds().addAll(data.visitedRoomIds());
+        p.setCurrentRoom(requireRoom(w, data.currentRoomId()));
         p.setGold(data.gold());
 
         // 4. Flags, quests, turn.