Can't re-enable FileVault after growing `system` partition
I decided to streamline the partitioning scheme as much as possible.  I deleted my leading, unused partition, but this still didn't make it go.

Somehow the partition type of the EFI partition had gotten changed, so I decided to fix that.  There doesn't seem to be any obvious way to do this from within OSX, so I rebooted off of a live Ubuntu boot image on a USB drive.  I used gdisk to set the type of the EFI partition to EF, and confirmed that this had also correctly changed its GUID to the correct one for that partition type.

For some reason, under linux the names of the EFI and Recovery partitions had disappeared, so I set them back to the names shown above.  I also confirmed that there was a FAT32 filesystem on the EFI partition.  Apparently the EFI filesystem is technically a different one from FAT32 but I left it as it was.

And it worked!  When I booted back to OSX, I ran Verify on the disk in Disk Utility, which recommended that I run Repair due to some "corruption" in the Recovery Partition, or perhaps just with its header.  I ran Repair, which reported success, and was then able to initiate full-disk FileVault encryption.