On a warm afternoon in Charlestown, the playground fills with laughter. A child steadies on a first bicycle as a parent calls out, “You’ve got it — keep pedaling!” Beneath tall shade trees, neighbors linger over lunch while sunlight filters through the leaves, scattering light across the grass. It feels like an ordinary neighborhood park. It is anything but ordinary. Beneath the brick walkways and green lawn of John Harvard Mall lies the story o f a young man who lived here f