feat(image): flash full disk images into LUKS; add Raspberry Pi OS

Support distributions that ship a full .img (Raspberry Pi OS, moode, RetroPie,
Manjaro ARM) rather than a rootfs tarball, by loop-mounting the image and
copying its boot + root partitions into a fresh LUKS container.

- distributions.yml/catalog.py/choosers.py: raspios catalog (lite64/desktop64/
  lite32 via the stable _latest redirects) + choose_raspios.
- plan.py: source_url override so a _latest redirect downloads under an .img.xz
  name that decompress_command recognises.
- transfer.py: transfer_disk_image (loop-mount -> repartition -> LUKS -> rsync
  copy with progress, cp fallback -> fix boot fstab); transfer_image gains
  interactive= and routes encrypted non-arch images here; download_image gains
  force_prompt.
- loopimg.py: losetup attach/detach/partition helper.
- fsutil.drop_fstab_mount + register_encrypted_root: replace a stock image's
  existing / fstab line instead of colliding with it.
- encryption.configure_encryption returns the onion address.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lim/distributions.yml

@@ -57,6 +57,19 @@ retropie_images:
     checksum: b5daa6e7660a99c246966f3f09b4014b
     image: retropie-buster-4.8-rpi4_400.img.gz
 
+# Raspberry Pi OS. The "_latest" endpoints 302-redirect to the current release;
+# image is only the local filename (must end .img.xz for decompress_command).
+raspios_images:
+  lite64:
+    source_url: https://downloads.raspberrypi.com/raspios_lite_arm64_latest
+    image: raspios_lite_arm64_latest.img.xz
+  desktop64:
+    source_url: https://downloads.raspberrypi.com/raspios_arm64_latest
+    image: raspios_arm64_latest.img.xz
+  lite32:
+    source_url: https://downloads.raspberrypi.com/raspios_lite_armhf_latest
+    image: raspios_lite_armhf_latest.img.xz
+
 # Extra kernel modules the initramfs needs for early network access,
 # see https://raspberrypi.stackexchange.com/questions/67051
 mkinitcpio_modules_by_rpi: