So: the Scout was not starting yesterday, after having been on the trickle charger since Tuesday. I tried it first thing this morning  after having been on the charger overnight, and got pretty much the same result—a chattering from the starter but not enough juice to keep it going. I started diagnosing by cleaning the contacts on the starter and all the positive wires, with no change. Then I tested the charge at the ignition wire on the starter to see if that was getting juice from the key, which it was. I pulled the battery and brought it to AutoZone to have them test it, which showed no problems. After a trip to the hardware store to buy some supplies, I rigged up a test jumper and bench-tested my two spare starters. The one that was making intermittent noise tested fine so I put the Scout up on jacks, pulled the tire, and swapped it out for the year-old unit.
Crucially, I disconnected the positive battery cable and put the battery on the trickle charger for the hour that took. After hooking it back up, the truck fired right up. I left everything in place, put the tire back on, dropped it onto the pavement, and cleaned up my tools. Then I went to start it up and move it—and had barely any juice.
So, a slow leak in the electrical system? Maybe a critter climbed up underneath on Wednesday afternoon and started munching on wires? Maybe the bulkhead connectors, which always have looked like they were blasted by Godzilla, finally melted?
The sky had turned gray around noon, and it began drizzling as I tried one last time to jump it from the Accord with no luck. So I used gravity and the gentle slope of the driveway to coast it back into the garage, disconnected the positive battery lead, and put the trickle charger on it one more time. Then I came inside and cracked a beer.