Oaklanders who frequent the waterfront paths along Lake Merritt are accustomed to seeing encampments and trash heaps. Last spring, though, they came upon a surprising sight. A small hand-built cabin suddenly appeared in a grassy clearing on the lake’s western shore, just a short walk from the swanky Lake Chalet restaurant. The structure looked as if it was made with permanence in mind. Sturdily built of measured wooden planks, with plexiglass windows and a corrugated steel roof, it was big…