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/nfpm.yaml

@@ -0,0 +1,65 @@
+name: fenris
+arch: amd64
+platform: linux
+version: "${VERSION}"
+maintainer: Fenris Maintainers <ops@bongbetic.com>
+description: >
+  NVMe wear monitor with persistent TUI — observes real-world drive use and
+  translates it into an understandable endurance outlook.
+homepage: https://git.bongbetic.com/xavierk/Fenris
+license: Proprietary
+
+depends:
+  - python3 (>= 3.10)
+  - smartmontools
+  - systemd
+
+contents:
+  # Staged tree: venv, wrapper, helpers, units, polkit, sysusers, tmpfiles
+  - src: build/stage/
+    dst: /
+    type: tree
+
+  # Configuration directory
+  - dst: /etc/fenris
+    type: dir
+    file_info:
+      mode: 0755
+
+  # Default placeholder-commented config (conffile for deb)
+  - src: packaging/fenris.conf
+    dst: /etc/fenris/fenris.conf
+    type: config
+    file_info:
+      mode: 0644
+
+  # Observation store directory — owned by package, never packed
+  # deb: created in postinst; rpm: %ghost
+  - dst: /var/lib/fenris
+    type: ghost
+    file_info:
+      mode: 2750
+
+scripts:
+  preinstall: packaging/preinst.sh
+  postinstall: packaging/postinst.sh
+  preremove: packaging/prerm.sh
+  postremove: packaging/postrm.sh
+
+overrides:
+  deb:
+    depends:
+      - python3 (>= 3.10)
+      - smartmontools
+      - systemd
+
+  rpm:
+    depends:
+      - python3 >= 3.10
+      - smartmontools
+      - systemd
+    scripts:
+      preinstall: packaging/preinst.sh
+      postinstall: packaging/rpm/post.sh
+      preremove: packaging/rpm/preun.sh
+      postremove: packaging/rpm/postun.sh