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/preinst.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+# preinst — abort if make-install remnants detected (spec §7, §9).
+# Dual marker: /var/lib/fenris/manifest.txt or /etc/systemd/system/fenris-collect.timer.
+set -eu
+
+MARKER1="/var/lib/fenris/manifest.txt"
+MARKER2="/etc/systemd/system/fenris-collect.timer"
+
+if [ -f "${MARKER1}" ] || [ -f "${MARKER2}" ]; then
+    echo >&2
+    echo >&2 "Fenris make-install remnants detected — refusing to install."
+    echo >&2
+    echo >&2 "Migrate to the package with:"
+    echo >&2 "  sudo make uninstall   # removes make-install files, preserves store + config"
+    echo >&2 "  sudo apt install fenris   # or: sudo dnf install fenris"
+    echo >&2
+    echo >&2 "See: https://git.bongbetic.com/xavierk/Fenris/blob/main/docs/spec/release-packaging.md#9-migration-from-make-install-systems"
+    echo >&2
+    exit 1
+fi