Add WatchEvents, enable/disable/weight RPCs, and config check

gRPC / proto
- Rename WatchBackendEvents → WatchEvents; return a stream of Event
  oneof (LogEvent, BackendEvent, FrontendEvent) with optional filter
  flags (log, log_level, backend, frontend)
- Add EnableBackend, DisableBackend, SetFrontendPoolBackendWeight RPCs
- Rename PauseResumeRequest → BackendRequest
- Add CheckConfig RPC returning ok/parse_error/semantic_error

maglevd
- Route slog through a LogBroadcaster (slog.Handler) so WatchEvents
  subscribers can receive structured log records independently of the
  daemon's own --log-level
- Add --reflection flag (default true) to toggle gRPC server reflection
- Add --check flag: validates config file and exits 0/1/2
- SIGHUP: use config.Check before applying reload; log parse vs semantic
  error separately; refuse reload on any error
- Rename default config path /etc/maglev → /etc/vpp-maglev

maglevc
- Add 'watch events [num <n>] [log [level <level>]] [backend] [frontend]'
  command; prints compact protojson, stops on any keypress or Ctrl-C;
  uses cbreak mode (not raw) so output post-processing is preserved
- Add 'set backend <name> enable|disable'
- Add 'set frontend <name> pool <pool> backend <name> weight <0-100>'
- Add 'config check' command

Debian packaging
- Rename service unit to vpp-maglevd.service
- Rename conffiles to /etc/default/vpp-maglev and /etc/vpp-maglev/
- Create maglevd system user/group in postinst; add to vpp group if present
- Add postrm; add adduser to Depends

debian/vpp-maglevd.service

@@ -0,0 +1,22 @@
+[Unit]
+Description=Maglev health-checker daemon
+Documentation=man:maglevd(8)
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+User=maglevd
+Group=maglevd
+EnvironmentFile=/etc/default/vpp-maglev
+ExecStart=/usr/sbin/maglevd --config ${MAGLEV_CONFIG}
+Restart=on-failure
+RestartSec=5s
+Type=simple
+
+# Grant only CAP_NET_RAW (needed for ICMP probes) and drop everything else.
+AmbientCapabilities=CAP_NET_RAW
+CapabilityBoundingSet=CAP_NET_RAW
+NoNewPrivileges=yes
+
+[Install]
+WantedBy=multi-user.target