feat(save): SaveData/SaveSlotInfo/SaveException model

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

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+package thb.jeanluc.adventure.save;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * JSON-serialised snapshot of the mutable game state — a delta laid over the
+ * world freshly loaded from YAML. Header fields ({@code slotName}, room,
+ * {@code turn}, {@code savedAtEpochMillis}) double as the slot-list metadata.
+ */
+public record SaveData(
+        int schemaVersion,
+        String worldTitle,
+        String slotName,
+        long savedAtEpochMillis,
+        int turn,
+        String currentRoomId,
+        List<String> visitedRoomIds,
+        int gold,
+        List<String> inventoryItemIds,
+        List<String> flags,
+        Map<String, Integer> questStages,
+        List<String> questCompleted,
+        Map<String, List<String>> roomItemIds,
+        Map<String, Boolean> switchStates
+) {
+    /** Current on-disk schema version. */
+    public static final int CURRENT_VERSION = 1;
+}

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

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+package thb.jeanluc.adventure.save;
+
+/** Thrown when a save cannot be written/read; carries a player-facing message. */
+public class SaveException extends RuntimeException {
+
+    public SaveException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+    public SaveException(String message) {
+        super(message);
+    }
+
+    /** Player-facing message (the constructor message is already user-friendly). */
+    public String getUserMessage() {
+        return getMessage();
+    }
+}

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

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+package thb.jeanluc.adventure.save;
+
+/** Lightweight metadata for one save slot, shown in the Load menu. */
+public record SaveSlotInfo(String slug, String slotName, String roomId,
+                           int turn, long savedAtEpochMillis) {
+}