Add GoVPP integration and GetVPPInfo gRPC call

VPP client (internal/vpp/)
- New package managing connections to both VPP API and stats sockets,
  treated as a unit: if either drops, both are torn down and
  re-established together.
- Run() loop: connect, fetch version via vpe.ShowVersion, read
  /sys/boottime from the stats segment, log vpp-connect, then monitor
  with control_ping every 10s. On failure, disconnect both, retry
  after 5s.
- Registers as client name "vpp-maglev" (visible in VPP's
  "show api clients").
- Flags: --vpp-api-addr (default /run/vpp/api.sock) and
  --vpp-stats-addr (default /run/vpp/stats.sock). Empty api addr
  disables VPP integration entirely.

gRPC / proto
- Add GetVPPInfo RPC returning VPPInfo: version, build_date,
  build_directory, pid, boottime_ns, connecttime_ns. Both times are
  unix timestamps in nanoseconds — the client computes durations
  locally for display.
- Returns codes.Unavailable if VPP is disabled or not connected.

maglevc
- Add 'show vpp info' command displaying version, build-date,
  build-dir, vpp-pid, vpp-boottime (with duration), and connected
  time (with duration).

tests/01-maglevd/maglevd-lab/start.sh

@@ -4,4 +4,4 @@ case "$ARCH" in
     x86_64)  ARCH=amd64 ;;
     aarch64) ARCH=arm64 ;;
 esac
-exec /opt/maglev/build/${ARCH}/maglevd --config /etc/maglev/maglev.yaml --log-level debug
+exec /opt/maglev/build/${ARCH}/maglevd --config /etc/maglev/maglev.yaml --vpp-api-addr "" --log-level debug