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_hook

@@ -0,0 +1,35 @@
+#!/bin/sh
+# initramfs-tools build hook: bake the Tor onion service into the initramfs.
+# Installed to /etc/initramfs-tools/hooks/tor by linux-image-manager.
+PREREQ="dropbear"
+prereqs() { echo "$PREREQ"; }
+case "$1" in
+    prereqs) prereqs; exit 0 ;;
+esac
+
+. /usr/share/initramfs-tools/hook-functions
+
+copy_exec /usr/bin/tor /usr/bin
+
+# glibc resolves the NTP server hostname via getaddrinfo(), which dlopen()s
+# these NSS modules at runtime; without them DNS silently fails and the clock
+# (RTC-less boards) stays at 1970, so Tor rejects the consensus.
+for nss in /usr/lib/*/libnss_dns.so.2 /usr/lib/*/libnss_files.so.2 \
+           /lib/*/libnss_dns.so.2 /lib/*/libnss_files.so.2; do
+    [ -e "$nss" ] && copy_exec "$nss"
+done
+
+# Full busybox for the ntpd applet (the initramfs busybox may lack it); a
+# distinct path keeps the initramfs's own busybox untouched.
+for bb in /bin/busybox /usr/bin/busybox; do
+    [ -x "$bb" ] && { copy_exec "$bb" /usr/local/bin/busybox; break; }
+done
+
+# Onion keys + torrc, staged on the system by lim.
+mkdir -p "${DESTDIR}/etc/tor/onion"
+for key in hostname hs_ed25519_public_key hs_ed25519_secret_key; do
+    cp -a "/etc/tor/initramfs-onion/${key}" "${DESTDIR}/etc/tor/onion/${key}"
+done
+chmod 0700 "${DESTDIR}/etc/tor/onion"
+chmod 0600 "${DESTDIR}/etc/tor/onion/hs_ed25519_secret_key"
+cp -a /etc/tor/initramfs-torrc "${DESTDIR}/etc/tor/torrc"