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/fenris.conf

@@ -0,0 +1,11 @@
+# Fenris configuration
+#
+# This file is managed by the fenris package. Local edits are preserved
+# across upgrades; changed defaults appear as .dpkg-new / .rpmnew.
+#
+# The device selector specifies which NVMe drive to monitor.
+# Uncomment and set exactly one device path:
+#
+#   devices = /dev/disk/by-id/nvme-Samsung_SSD_980_PRO_500GB_S5PANS0T123456
+#
+# See https://git.bongbetic.com/xavierk/Fenris for documentation.