So... I wrecked my phone last night. :-(
Android's phone encryption feature has been around for aaaages, so I was confident when I started the encryption process last night. Unfortunately, something went wrong, and last night I discovered that everything was toast.
After a bit of digging, I found someone else with exactly the same problem!
"unable to boot into my phone as it sits at the boot screen. When I try to boot into TWRP, it asks for my password. [...] it appears to decrypt the partition and mount, but then …