feat(core): add player profile persistence (position, inventory, stats)

core/include/vf/PlayerData.h

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+#include "Codec.h"
+#include "Item.h"
+#include "Math.h"
+#include "Nbt.h"
+#include "Survival.h"
+
+namespace vf {
+
+// Everything that persists for one player between sessions: transform, the
+// survival vitals, experience, the selected hotbar slot and the full item
+// inventory (block items, materials, food and tools with durability).
+struct PlayerData {
+    Vec3 position;
+    float yaw = 0.0f;
+    float pitch = 0.0f;
+    SurvivalState survival;
+    int xpLevel = 0;
+    float xpProgress = 0.0f;
+    int selectedSlot = 0;
+    std::vector<ItemSlot> inventory;
+};
+
+NbtTag playerToNbt(const PlayerData& player);
+bool playerFromNbt(const NbtTag& root, PlayerData& out);
+
+// NBT-backed byte serialization, round-tripping through the existing tag tree.
+ByteBuffer savePlayer(const PlayerData& player);
+bool loadPlayer(const uint8_t* data, std::size_t size, PlayerData& out);
+
+} // namespace vf