allow caller-defined allocation strategies for WorldSection

[KaptainWutax]

This allows WorldSection acquisition to use caller-defined allocation strategies while preserving the existing default behavior and the public API surface. It uses MemorySegment instead of long[] to allow for off-heap + safe native access to the data without JNI. This also moves the implementation toward Java's supported FFM API, as the memory-access methods in Unsafe were deprecated for removal in JDK 23.

WorldSection.Allocator myCustomAllocator = new WorldSection.Allocator() {
    @Override
    public MemorySegment allocate(int lvl, int x, int y, int z, int size) {
        return MemorySegment.ofArray(new long[size]);
    }

    @Override
    public void release(MemorySegment segment) {
        // Leave it to the GC
    }
};

// Existing behavior
WorldSection section = engine.acquire(lvl, x, y, z, false);

// Custom allocator
WorldSection section = engine.acquire(lvl, x, y, z, false, myCustomAllocator);

[KaptainWutax]

This approach isn't the best fit for the project. Closing this as superseded.