feat(state): add validated persistent game state

src/main/java/games/dmg/spigottyrant/PersistentState.java

@@ -0,0 +1,20 @@
+package games.dmg.spigottyrant;
+
+import java.util.Map;
+import java.util.UUID;
+
+public record PersistentState(GameState game, Map<UUID, PlayerState> players) {
+    public PersistentState {
+        game = game == null ? GameState.empty() : game;
+        players = players == null ? Map.of() : Map.copyOf(players);
+        for (Map.Entry<UUID, PlayerState> entry : players.entrySet()) {
+            if (!entry.getKey().equals(entry.getValue().playerId())) {
+                throw new IllegalArgumentException("player map key must match player state ID");
+            }
+        }
+    }
+
+    public static PersistentState empty() {
+        return new PersistentState(GameState.empty(), Map.of());
+    }
+}