A lawn earns its keep on a Saturday in July, when someone drives a stake into it and the yard becomes a field. Triangle Lawn Games - Boston keeps Gloucester grass in the condition that makes that possible — cut low and level, thin spots coaxed back, no ankle-turning surprises out where the game happens. Down at Stage Fort Park the ballfields run almost to the water and generations have played with the harbor in view; a backyard deserves the same honest footing. Mow, trim, edge, tidy up, then step back and let the afternoon use it. That is the whole ambition, and it is a good one.