Or, more accurately, I suppose he tried to climb out in the total darkness and fell. He didn't cry much, seeming more stunned than anything. After checking to make sure there were no noticeable injuries, I brought him into bed with us for the rest of night. He fell back asleep eventually, while I lay there making plans to deal with this latest development.
Those plans were implemented yesterday, by A. taking the crib out and hauling out one of our multiple spare twin-sized bed frames from the barn. He also put locks on the doors in Cubby's room that lead to the bathroom and the Secret Stair and removed the extremely heavy and tippy space heater (hello, incredibly un-child-friendly bedroom!).
I braced myself for several days of no naps and difficult nights. Yesterday I lay down with him in his bed for his first nap in it, and he was asleep within ten minutes. After which I snuck out and left him to it. It was a fairly short nap, but at least he did sleep by himself.
Last night I spent an hour figuring out the best way to get him to sleep. Turns out, he has no need of my physical presence even near the bed. After some trial and error, I eventually ended up just standing in the darkened doorway to his room and every time he'd try an experimental, "Mommy?" to make sure I was still there, I'd whisper it was time to sleep. Which he did, eventually.
He slept nine hours last night by himself in his bed. That might have been due to total exhaustion, but I prefer to see it as a hopeful sign that Cubby will adapt to his new sleeping arrangement without too much drama.
Now watch. Bet he won't sleep AT ALL tonight. But at least he won't be swan diving from four feet up in a desperate bid for freedom. That's an improvement.