So much for intuition. I decided to leave the planter with the chrysalis on it outside until the butterfly revealed itself, and this morning I stepped out on the front porch to find it gone. All that was left of it was one thin green section lying near a couple dark splotches on the pavement that were covered with ants; I assume these are the caterpillar’s remains.
Ah well. There’s always next year. I spotted another monarch caterpillar in the garden earlier in the week, so I’ll look around more closely out there to see if I can find its chrysalis.