feat(core): add chunk load/unload radius streaming

core/include/vf/Streaming.h

@@ -0,0 +1,59 @@
+#pragma once
+
+#include <cstdint>
+#include <unordered_set>
+#include <vector>
+
+namespace vf {
+
+// A chunk column address (x, z); a column owns the full vertical stack of chunks.
+struct ColumnCoord {
+    int x = 0;
+    int z = 0;
+    bool operator==(const ColumnCoord& o) const { return x == o.x && z == o.z; }
+};
+
+struct ColumnCoordHash {
+    std::size_t operator()(const ColumnCoord& c) const {
+        std::uint64_t h = 1469598103934665603ull;
+        h ^= static_cast<std::uint32_t>(c.x); h *= 1099511628211ull;
+        h ^= static_cast<std::uint32_t>(c.z); h *= 1099511628211ull;
+        return static_cast<std::size_t>(h);
+    }
+};
+
+struct StreamingConfig {
+    int loadRadius = 8;     // columns within this circle are kept resident
+    int unloadRadius = 10;  // columns are only evicted past this larger circle
+};
+
+struct StreamDelta {
+    std::vector<ColumnCoord> load;    // to generate, nearest-to-player first
+    std::vector<ColumnCoord> unload;  // now beyond the unload ring
+};
+
+// Tracks which chunk columns are resident around a moving centre. The load and
+// unload rings differ (hysteresis) so a player pacing across a boundary does not
+// thrash the same columns in and out every step.
+class ChunkStreamer {
+public:
+    explicit ChunkStreamer(StreamingConfig config = {}) : config(config) {}
+
+    // Recomputes residency around `centre`, mutating the resident set and
+    // returning the columns that entered (to load) and left (to unload).
+    StreamDelta update(ColumnCoord centre);
+
+    bool isLoaded(ColumnCoord column) const { return resident.count(column) != 0; }
+    std::size_t loadedCount() const { return resident.size(); }
+    const StreamingConfig& settings() const { return config; }
+
+private:
+    StreamingConfig config;
+    std::unordered_set<ColumnCoord, ColumnCoordHash> resident;
+};
+
+// Columns whose centre lies within `radius` of (0,0), sorted nearest first.
+// Distance is Euclidean so the loaded area is a disc, not a square.
+std::vector<ColumnCoord> columnsInRadius(ColumnCoord centre, int radius);
+
+} // namespace vf