Implement XBPS packaging lifecycle for Void Linux (issue #85)

Add native XBPS package support with runit service lifecycle:
- packaging/xbps/install.sh: migration guard, fresh install (dormant),
  upgrade (snapshot, migrate, config preservation)
- packaging/xbps/remove.sh: sanctioned disable, purge (full cleanup)
- Makefile: package-xbps target with dependencies and config files
- tests/test_packaging.py: 5 XBPS-specific tests + updated shared tests

Acceptance criteria addressed:
- Fresh install remains dormant; runit down marker set
- Upgrade snapshots and migrates observation history safely
- Removal performs sanctioned pause and retains history
- Migration guard blocks install over make-install remnants
- Debian/RPM regressions verified via existing shared tests

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

.gitignore

@@ -19,3 +19,4 @@ src/fenris.egg-info/
 .pytest_cache/
 .venv/
 MagicMock*
+<MagicMock*