The veterinarian just called with the results of the pathology report on Baxter's lump -- it was BENIGN! The tumor turned out to be a hemangioma (not a hemangiosarcoma, which was the worst case scenario), and the removal of it is curative. He said they got it all and there's no reason to expect it would return. While it looks considerably different than a lipoma (it's a blood vessel tumor) it's just one of those random lumps dogs sometimes get.
Needless to say, we are all very relieved around here. Baxter, who has seemed entirely back to his normal, earnest, goofy self for the past several days, has no idea why I keep showering him with extra hugs and kisses this morning. But he seems fine with it.