The router was fine. That is the whole tragedy. It had been sitting on the shelf doing its job for two years, never dropped a connection, never needed a reboot. So naturally I decided it would be better with OpenWrt on it.
The flash went in cleanly, the router came back up, and then on the second reboot it didn't. No DHCP, no ping, no lights doing anything reassuring. I had managed to brick a device that had committed no crime other than being within reach on a quiet Friday.
The recovery, as ever, was the soldering iron and a USB-to-serial adapter. Three pads on the board, 3.3V logic, 115200 baud, and a bootloader that was still alive even though the rest of the firmware was not. From there it was a TFTP recovery image and ten minutes of holding my breath. It came back.
I did get OpenWrt on it in the end, and it is genuinely nicer, but I can't honestly claim the upgrade was worth the evening. Lesson noted, lesson ignored next month, probably. If it isn't broken, leave it alone, or at least leave the serial header soldered on first.