pixelsandbox package

Package pixelsandbox is a deliberately generic/throwaway practice deployment of the GNO Pixels design -- not the final intended realm name, so redeploying while the design is still changing (which it will be, more than once, on the way to something we're satisfied with) never collides with whatever the real, permanent realm ends up being called.

Design itself matches this project's local pixelgame3 iteration: expandable bounds instead of a fixed size, per-pixel provenance (who placed it, at what block), and an owner-gated import hook for a possible future migration to a fresh, permanently-named deployment.