Post-post-surgical update
tl;dr: tumor margins were clear, but they found cancer in one out of three lymph nodes sampled.
Long version: This afternoon I talked with Dr. Greer (the backup surgeon while Dr. Chu is out). She gave me an overview of the pathology report from my surgery, as well as sending me the text version that is full of interesting bits of technical jargon. The tumor removal appears to have been successful: "negative margin" meaning the outer perimeter of tissue was all cancer-free, including a couple of extra bits Dr. Chu removed probably on suspicion of how they looked. Three lymph nodes were removed, one of which had cancerous cells (contained -- no extranodal extension).
Dr. Greer has referred me to medical oncology, where she suspects they will want to do a CAT scan plus radiation treatment on both the breast and axillary lymph node regions, and may also recommend chemotherapy and/or hormone therapy. They should be calling in the next day or two to get the ball rolling. She also said they will probably not order oncotype testing because the cancer has already started spreading so it may not add much new information -- it's apparently more useful when trying to determine the risk of spread.
We also chatted a bit about the surgery recovery. I'm doing pretty well -- we've been walking every day, and today we even tried out a brief slow bike ride (verdict: mostly ok for a short stretch! Mainly a bit too bumpy, so I don't think I'll be biking much distance for a little longer). The lymph node incision is much slower to heal and is still a bit swollen, but I'm off painkillers and have energy for many normal activities. More on that below.
Wading into the jargon of the report, it's been interesting to google a bit and learn some new words. The tumor is described as being single-focus (rather than multiple centers), histologic and nuclear grade 3 (most abnormal cells/most indication of rapid cell division), cribriform and comedone (sort of swiss-cheese like with necrotic cells that are residual and may collect things like calcium deposits from the blood). Most surface-level searches refer to comedonecrosis in the context of noninvasive cancers where these calcium deposits can be a warning sign of increased malignancy. Not sure that's of much use in the case where the cancer has started to spread, but it does go along with the general portrait I'm getting of this one. They haven't given it a final stage yet, other than "not stage 1."
So. Feelings?
I guess... overall I'm feeling grateful we've been moving things along quickly, because it feels like we maybe caught this in the nick of time. A little earlier would have been better of course, but only one out of three lymph nodes seems to imply it had only just started to spread, and maybe we got it all out. Fingers crossed. I'm not really looking forward to the next phase of treatment or a lifetime of hyper-surveillance, since it definitely seems like the sort of cancer that's inclined to pop up again and calls for more vigorous treatment options, but I'm glad there isn't another surgery needed, and I'm really glad the surgery recovery has been as smooth as I could have asked for. It has also honestly been really great to turn off the normal stressors for a bit and just unwind, read some books, get up at 5 AM to put some bread puddings in the oven for a bunch of Morris dancers*, and wear some silly hats. 
Much love to all who have sent various forms of well-wishes but especially those who have sent cookies.
*Merry May! Looks like they managed to get the sun up after all. It was a bit of a near thing -- I feel a little culpable for not showing up for sunrise this year, but a girl needs her boob rest.