feat(core): add prefab structure templates and block stamping

core/include/vf/Structures.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <cstdint>
+#include <vector>
+#include "World.h"
+
+namespace vf {
+
+enum class StructureKind : uint8_t {
+    OakTree = 0,
+    PineTree = 1,
+    Hut = 2,
+};
+
+struct StructureEdit {
+    Vec3i offset;   // relative to the structure base (first cell above ground)
+    Block block;
+};
+
+// Deterministic 32-bit mix of a seed and a coordinate; used to seed per-structure
+// variation so the same seed and position always yield the same build.
+uint32_t structureHash(uint32_t seed, int x, int y, int z);
+
+// Builds the block edits for a structure. Pure: identical (kind, seed) always
+// produce the identical edit list, in the same order.
+std::vector<StructureEdit> buildStructure(StructureKind kind, uint32_t seed);
+
+// Writes edits into the world at base + offset. Leaves yield to existing solid
+// blocks (they only fill air); trunk, planks, roof and the lantern overwrite.
+void stampStructure(World& world, const Vec3i& base, const std::vector<StructureEdit>& edits,
+                    int maxY);
+
+} // namespace vf