with a tentative netinst for amd64, I deployed a laptop, full
disk, switching the / FS from the default ext4 to btrfs, and proceeding
as always for the rest.
Then I broke the boot by removing the initramfs.
Then ran d-i in (graphical) rescue mode, picked the right partition (I
had no idea which, but I could give it several tries), then executed a
shell in the installed system, `update-initramfs -c -k all` which was
required, and `update-grub` for peace of mind, probably not required.
And the system boots again.
I'm not going to try and compare that with earlier images, but that
looks to be “not entirely broken”. If that confirms what people have
been working on, yay.