feat(image): distro-agnostic remote unlock via initramfs backends + Debian support

Split the mkinitcpio-only remote-LUKS-unlock path into an InitramfsBackend
ABC with a get_backend() dispatch, and add the initramfs-tools backend for
Debian / Raspberry Pi OS.

- base.py: six-step backend contract; encryption.py becomes a thin,
  distro-neutral sequencer (get_backend by distribution).
- initramfs_tools.py: crypttab `none luks,initramfs`, cmdline rewritten to
  root=/dev/mapper + ip=::::host:eth0:dhcp, dropbear-initramfs
  authorized_keys, update-initramfs -k all (no build-host uname leak).
- shipped hooks (configuration/initramfs-tools/*): single-hop non-anonymous
  onion, libnss DNS baking, sed-not-source DHCP, kill-tor-before-pivot.
- shared offline onion keygen in keygen.py; tor.py removed (logic moved to
  mkinitcpio.py).
- raspios added to the apt distro family (session.py, raspberry.py).

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

lim/configuration/packages/server/luks-debian.txt

@@ -0,0 +1,5 @@
+# Packages for LUKS remote unlock on Debian / Raspberry Pi OS (initramfs-tools)
+cryptsetup
+cryptsetup-initramfs
+dropbear-initramfs
+busybox

lim/configuration/packages/server/tor-debian.txt

@@ -0,0 +1,3 @@
+# Packages for the Tor onion unlock service in the initramfs (Debian)
+tor
+busybox  # full applet set: ntpd for clock sync on RTC-less boards