I was scolded (rightly so!) in a comment, for not updating Murphy's fan club about his status!
Murphy's back to normal. My checking account is missing $254, but he's his old bossy self. He hasn't needed any more pain meds, and he's eating and walking and being himself. The vet has no idea what Thursday's drama was all about, and if it recurs she wants to refer him to an internal medicine vet, but he's back to normal, so we'll cross that bridge later.
I have no clue. I've been dealing with his condition since 2009, and I'm not an alarmist who rushes him to the doctor over every little change, but Thursday he definitely was a hurtin' little puppy. He's recovered, and the vet is the first to say she has no idea why and doesn't take credit for fixing anything. Whatever it was, it passed.
It was not a totally wasted vet visit, because we do have new blood work and urinalysis, and other than the drop in albumin, his test results were within normal range. The drop is the disease; we can tinker with it but we can't cure it. But he's his happy little bossy self again, and that's all that really matters.