Round II of flushing the tank and mopping it empty. God damn. I had miserable visions of being unable to fix the toilet and having to pee in the bushes tonight and call R in the morning, who probably knows as little about plumbing as I do, but he has a penis, and I hear that makes a difference in fix-it-abilities.
Argh. I removed the new rig again, lay it flat on the floor next to the old one and stared at them for a while, comparing and contrasting. Looked at the instructions again. Turns out I'd skipped a rather important step -- Clover, it seems, had absconded with a rubber ring that was crucial in the sealing process. I retrieved it, fortunately none the worse for wear; she snarled and whined, and I, again, installed the new Fluidmaster Toilet Tank Fill Valve, this time with rubber ring. I hooked, tightened, doublechecked, turned the water on cautiously...
And it works! Magically, when I push the lever down, the old poopy water disappears from the bowl, and it is replaced by nice fresh water. Then, water runs into the tank, and it stops when it's full. Holy crap.
The whole ordeal took about an hour and a half. I now know toilets inside and out. I am a Toilet Expert. I am a superhero!
Then a light bulb in my kitchen burned out. I changed that too, but it wasn't as exciting.