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>

Makefile

@@ -280,6 +280,9 @@ XBPS_SIGNING_KEY ?= $(HOME)/.ssh/id_xbps
 
 package-xbps: stage
 	@echo "=== Building XBPS package ==="
+	cp packaging/xbps/install.sh build/stage/INSTALL
+	cp packaging/xbps/remove.sh build/stage/REMOVE
+	chmod 755 build/stage/INSTALL build/stage/REMOVE
 	xbps-create -A x86_64 \
 		-n fenris-$(FENRIS_VERSION)_1 \
 		-s "Fenris NVMe wear monitor" \
@@ -287,6 +290,8 @@ package-xbps: stage
 		-m "Fenris Packaging <packaging@bongbetic.com>" \
 		-H "https://git.bongbetic.com/xavierk/Fenris" \
 		-l "MIT" \
+		-D "python3>=3.10 smartmontools" \
+		-F "/etc/fenris/fenris.conf" \
 		build/stage
 	@echo "=== XBPS package built: fenris-$(FENRIS_VERSION)_1.x86_64.xbps ==="