More likely to be a seedling than a runner at that distance with no plants showing in between. Raspberries travel by increments not 50 foot jumps. Unless of course you've mown down all the intervening canes without noticing them. And there's no reason you shouldn't eat them if they are definitely raspberries.

How close is the rogue to any neighbor's raspberries?

Good point. It might be a runner from beyond the garden.

Birds "deposit" seeds all over. This is how any unmowed portion of my yard suddenly has wineberries springing up

Can't be certain but I'm pretty sure the neighbors on that side do not have raspberries or a garden.

I suppose it could have come from seeds deposited by birds or other critters; I'm just surprised it's managed to successfully grow and flourish in a spot that has been completely shielded by direct sunlight, and now gets about an hour, maybe, at most.

Ate a few berries yesterday. Should make for a nice snack break while I tend to the non-"wild" sections of the garden.

Former elderly neighbor had a successful red raspberry patch directly under an ailanthus tree and by a fence, in deep shade, for many years.