The vet's report was that the lump was pretty encapsulated, but that inside looked possibly like necrotic tissue that definitely needs to be analyzed. The report should be back from the lab early next week. She did think that she got all the visible tumor, but said that if this should be a fibrosarcoma, it could've had microscopic tendrils that are impossible to trace and excise. Doing a little googling, it appears that cats fairly often get fibrosarcomas induced by vaccinations, and this lump is certainly in a likely spot to be that. We'll see when the biopsy results come back.
In the meantime, I have 4 syringes of pain medicine to squirt into Agatha's mouth the next 4 mornings, and she'll go back in 10 days to have the staples removed. That will be the day before I leave for a week in Denver, so I'm hoping her recovery goes smoothly.