feat(core): declare save-format v3 world and entity serialization interface

core/include/vf/SaveV3.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <cstdint>
+#include <vector>
+#include "World.h"
+#include "Entity.h"
+
+namespace vf {
+
+// A rectangular byte grid: two parallel channels over an origin/dims box. Used to
+// carry a fluid snapshot (levels + sources) or a light snapshot (sun + block).
+struct GridSnapshot {
+    Vec3i origin;
+    Vec3i dims;
+    std::vector<uint8_t> channelA;
+    std::vector<uint8_t> channelB;
+};
+
+// The full v3 save payload: world blocks plus the simulation state that v1/v2 did
+// not persist (mobs, fluid, light).
+struct SaveV3 {
+    World world;
+    std::vector<Mob> mobs;
+    bool hasFluid = false;
+    GridSnapshot fluid;
+    bool hasLight = false;
+    GridSnapshot light;
+};
+
+// Binary format "VXF3": magic, version 3, the v1 RLE block stream, then tagged,
+// length-prefixed sections (mobs, fluid, light). Unknown sections are skipped.
+std::vector<uint8_t> serializeSaveV3(const SaveV3& save);
+
+// Loads a save into `out`. Accepts a v3 buffer (full state) or a legacy v1/v2
+// buffer (magic "VXF1"), in which case only the world is populated and the
+// mobs/fluid/light stay empty. Returns false on bad magic or a truncated stream.
+bool deserializeSaveV3(const uint8_t* data, std::size_t size, SaveV3& out);
+
+} // namespace vf