feat(packaging): nfpm-based deb + rpm build infrastructure (spec §3-7, ADR 0007)

Implement the packaging configuration, staging script, maintainer scripts,
and container test harness for building native deb and rpm packages.

Core files:
- packaging/nfpm.yaml: single source of truth for both formats
- packaging/stage.sh: builds staged tree (venv, wrapper, helpers, units, polkit, sysusers, tmpfiles)
- packaging/fenris.conf: placeholder-commented default configuration
- packaging/postinst.sh, prerm.sh, postrm.sh: POSIX-compatible deb maintainer scripts
- packaging/rpm/post.sh, preun.sh, postun.sh: RPM scriptlets
- packaging/sysusers.d/fenris.conf, tmpfiles.d/fenris.conf: systemd fragments
- packaging/fenris.repo: dnf consumer setup
- packaging/keys/fenris-packaging.asc: public key placeholder

Build targets added to Makefile: stage, package-deb, package-rpm, package, release, clean.
Container test harness in tests/test_packaging.py covering dormant install,
migration guard, upgrade semantics, and removal semantics across the
compatibility matrix (Debian 12, Ubuntu 22.04/24.04, Fedora 40).
Dormant CI workflow at .gitea/workflows/release.yml.

All 289 existing tests pass without regression.

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

packaging/rpm/postun.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+# RPM %postun — post-uninstall scriptlet (spec §7).
+#
+# On erase ($1 -eq 0): remove config (unmodified removed, modified as .rpmsave),
+#                       store, backups, and group.
+# On upgrade ($1 -ge 1): daemon-reload only.
+set -eu
+
+if [ "$1" -eq 0 ]; then
+    # Package fully erased
+    rm -rf /etc/fenris
+    rm -rf /var/lib/fenris
+    # Remove the fenris group if it exists
+    if getent group fenris > /dev/null 2>&1; then
+        groupdel fenris 2>/dev/null || true
+    fi
+fi
+
+# Always daemon-reload (units may have been removed)
+systemctl daemon-reload 2>/dev/null || true