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/initramfs-tools/tor_bottom

@@ -0,0 +1,19 @@
+#!/bin/sh
+# initramfs-tools cleanup hook: stop Tor before the pivot to the real root.
+# Installed to /etc/initramfs-tools/scripts/init-bottom/tor by lim.
+# Nothing from the initramfs may keep running once run-init replaces it.
+PREREQ=""
+prereqs() { echo "$PREREQ"; }
+case "$1" in
+    prereqs) prereqs; exit 0 ;;
+esac
+
+# pidof/killall may be absent; scan /proc for the tor binary (no extra tools).
+for pid in $(ls /proc 2>/dev/null); do
+    case "$pid" in
+        *[!0-9]*) continue ;;
+    esac
+    [ "$(readlink "/proc/$pid/exe" 2>/dev/null)" = "/usr/bin/tor" ] \
+        && kill "$pid" 2>/dev/null
+done
+exit 0