Then I poured a half-cup of bleach into the water heater. Take that, smelly water! Anything that might be in there that thrives on hot water and iron oxide (and excretes hydrogen sulfide) should be dead now.
Once it was out I hied m'self to Home Depot for a flexible anode, because I could not fit the spare in with the space I had; then after stopping at Marnell's for beef sandwiches I got to work in the basement.
Now, some years ago, originally, there was a little 3x3 hole in the wall which gave access to the water heater. When one was replaced (not sure if the original or a later one) someone took a Sawzall to the wall to get it out, because the wall was not load-bearing and there was no reason not to make the hole bigger. Well, I enlarged it a bit more to see if I could get the spare I had on hand into the water heater, but even getting creative with bending it and then unbending it as it went in didn't help, so I discarded that plan and made with the $25 replacement from Home Depot.
Got it in, got it torqued down, repressurized the system--no leaks. I advised Mrs. Fungus not to run any hot water for a while as I want the bleach to have a chance to work, just in case some kind of bacteria is causing the stink. Fortunately, if it is, it's not going to care much for sea-level pressure and room temperature; extremophile bacteria are pretty fussy about their living conditions, just like we are. (This would also explain why the smell went away for about two weeks after the first time I drained it: draining the water heater wiped out most of the colony and it took that long for it to come back.)
Regardless, I can now change the anode with impunity, but I don't expect that I'll need to.
Step 2, cut grass. Got that done like a boss.
Step 3: enjoy the rest of my weekend with the knowledge of a job well done.