feat(core): add per-chunk random tick sampling for growth and decay

core/include/vf/RandomTick.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <cstdint>
+#include <vector>
+#include "Math.h"
+
+namespace vf {
+
+// Deterministic random-tick position selector. For a chunk coordinate and an
+// absolute world tick it yields a repeatable list of local cell positions,
+// seeded purely by (worldSeed, chunkCoord, tick, index). Two selectors built
+// with the same seed produce identical streams, so the same simulation replays
+// bit for bit. Draws are independent per index and may repeat, matching how a
+// real per-chunk random tick samples with replacement.
+class RandomTick {
+public:
+    RandomTick(uint32_t worldSeed = 1u, int ticksPerChunk = 3);
+
+    int ticksPerChunk() const { return ticks; }
+    uint32_t worldSeed() const { return seed; }
+
+    void sampleLocal(const Vec3i& chunkCoord, long long tick, std::vector<Vec3i>& out) const;
+    std::vector<Vec3i> sampleLocal(const Vec3i& chunkCoord, long long tick) const;
+
+    void sampleGlobal(const Vec3i& chunkCoord, long long tick, std::vector<Vec3i>& out) const;
+    std::vector<Vec3i> sampleGlobal(const Vec3i& chunkCoord, long long tick) const;
+
+    // Raw 64-bit value for (chunkCoord, tick, index) under this seed. Callers can
+    // roll their own seeded events off the same stream without touching layout.
+    uint64_t hash(const Vec3i& chunkCoord, long long tick, uint32_t index) const;
+
+private:
+    uint32_t seed;
+    int ticks;
+};
+
+} // namespace vf