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/image/initramfs/initramfs_tools.py

@@ -68,6 +68,8 @@ class InitramfsToolsBackend(InitramfsBackend):
             "  defaults,noatime  0  1"
         )
         ui.info(f"Configuring {fstab_path}...")
+        # A stock image already mounts / by PARTUUID; drop it so the mapper wins.
+        fsutil.drop_fstab_mount("/", fstab_path)
         fsutil.ensure_line_in_file(fstab_line, fstab_path)
 
         crypttab_path = root / "etc/crypttab"