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).

cmd/maglevd/main.go

@@ -23,6 +23,7 @@ import (
 	"git.ipng.ch/ipng/vpp-maglev/internal/config"
 	"git.ipng.ch/ipng/vpp-maglev/internal/grpcapi"
 	"git.ipng.ch/ipng/vpp-maglev/internal/metrics"
+	"git.ipng.ch/ipng/vpp-maglev/internal/vpp"
 )
 
 func main() {
@@ -40,6 +41,8 @@ func run() error {
 	configPath := stringFlag("config", "/etc/vpp-maglev/maglev.yaml", "MAGLEV_CONFIG", "path to maglev.yaml")
 	grpcAddr := stringFlag("grpc-addr", ":9090", "MAGLEV_GRPC_ADDR", "gRPC listen address")
 	metricsAddr := stringFlag("metrics-addr", ":9091", "MAGLEV_METRICS_ADDR", "Prometheus /metrics listen address (empty to disable)")
+	vppAPIAddr := stringFlag("vpp-api-addr", "/run/vpp/api.sock", "MAGLEV_VPP_API_ADDR", "VPP binary API socket path (empty to disable)")
+	vppStatsAddr := stringFlag("vpp-stats-addr", "/run/vpp/stats.sock", "MAGLEV_VPP_STATS_ADDR", "VPP stats socket path")
 	logLevel := stringFlag("log-level", "info", "MAGLEV_LOG_LEVEL", "log level (debug|info|warn|error)")
 	flag.Parse()
 
@@ -92,13 +95,20 @@ func run() error {
 		}
 	}()
 
+	// ---- VPP connection -----------------------------------------------------
+	var vppClient *vpp.Client
+	if *vppAPIAddr != "" {
+		vppClient = vpp.New(*vppAPIAddr, *vppStatsAddr)
+		go vppClient.Run(ctx)
+	}
+
 	// ---- gRPC server --------------------------------------------------------
 	lis, err := net.Listen("tcp", *grpcAddr)
 	if err != nil {
 		return fmt.Errorf("listen %s: %w", *grpcAddr, err)
 	}
 	srv := grpc.NewServer()
-	maglevServer := grpcapi.NewServer(ctx, chkr, logBroadcaster, *configPath)
+	maglevServer := grpcapi.NewServer(ctx, chkr, logBroadcaster, *configPath, vppClient)
 	grpcapi.RegisterMaglevServer(srv, maglevServer)
 	if *enableReflection {
 		reflection.Register(srv)