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/loopimg.py

@@ -0,0 +1,28 @@
+"""Attach a stock disk image as a loop device to read its partitions."""
+
+from pathlib import Path
+
+from lim import runner, ui
+from lim.errors import LimError
+
+
+def attach(image_path: Path) -> str:
+    """Attach the image via losetup with partition scanning; return /dev/loopN."""
+    ui.info(f"Attaching {image_path} as a loop device...")
+    loop = runner.output(
+        ["losetup", "-Pf", "--show", str(image_path)],
+        sudo=True,
+        error_msg=f"Attaching {image_path} as a loop device failed.",
+    ).strip()
+    if not loop:
+        raise LimError(f"losetup returned no device for {image_path}.")
+    return loop
+
+
+def detach(loop: str) -> None:
+    runner.run(["losetup", "-d", loop], sudo=True, check=False)
+
+
+def partition(loop: str, number: int) -> str:
+    """/dev/loop0 -> /dev/loop0p1 (losetup -P names partitions with a p suffix)."""
+    return f"{loop}p{number}"