feat(core): add chunk dirty-tracking and network sync batching

core/include/vf/ChunkSync.h

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <cstddef>
+#include <set>
+#include <unordered_map>
+#include "Chunk.h"
+#include "Math.h"
+#include "Protocol.h"
+#include "World.h"
+
+namespace vf {
+
+// Server side of chunk replication: records which block slots in each chunk
+// changed since the last flush and turns them into wire packets.
+class ServerChunkSync {
+public:
+    void markDirty(const Vec3i& chunkCoord, int x, int y, int z);
+    void markDirtyIndex(const Vec3i& chunkCoord, int localIndex);
+
+    std::size_t dirtyCount(const Vec3i& chunkCoord) const;
+    bool anyDirty() const;
+    void clear(const Vec3i& chunkCoord);
+    void clearAll() { dirty.clear(); }
+
+    // A full chunk snapshot for the initial send; false when the chunk is absent.
+    bool buildFull(const World& world, const Vec3i& chunkCoord, ChunkData& out) const;
+
+    // An incremental update from the dirty set; false when nothing is dirty.
+    bool buildDelta(const World& world, const Vec3i& chunkCoord, MultiBlockChange& out) const;
+
+    // Whether a full resend is cheaper than a per-block delta at this dirty size.
+    bool shouldResendFull(const Vec3i& chunkCoord, std::size_t threshold) const;
+
+private:
+    std::unordered_map<Vec3i, std::set<int>, Vec3iHash> dirty;
+};
+
+// Client side: applies received packets into a local world mirror.
+class ClientChunkSync {
+public:
+    void applyFull(const ChunkData& packet, World& world);
+    void applyDelta(const MultiBlockChange& packet, World& world);
+    void applyBlockChange(const BlockChange& packet, World& world);
+};
+
+} // namespace vf