feat(board): add readable physical board signs

src/main/java/games/dmg/spigotquestboard/BoardRegistry.java

@@ -46,6 +46,10 @@ final class BoardRegistry {
         return boards.size();
     }
 
+    synchronized Map<BoardId, RegisteredBoard> registeredBoards() {
+        return Map.copyOf(boards);
+    }
+
     private static Map<BoardId, RegisteredBoard> index(BoardState state) throws IOException {
         Map<BoardId, RegisteredBoard> indexed = new LinkedHashMap<>();
         for (RegisteredBoard board : state.boards()) {