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).

internal/grpcapi/server.go

@@ -13,6 +13,7 @@ import (
 	"git.ipng.ch/ipng/vpp-maglev/internal/checker"
 	"git.ipng.ch/ipng/vpp-maglev/internal/config"
 	"git.ipng.ch/ipng/vpp-maglev/internal/health"
+	"git.ipng.ch/ipng/vpp-maglev/internal/vpp"
 )
 
 // Server implements the MaglevServer gRPC interface.
@@ -22,16 +23,17 @@ type Server struct {
 	checker    *checker.Checker
 	logs       *LogBroadcaster
 	configPath string
+	vppClient  *vpp.Client // nil when VPP integration is disabled
 }
 
 // NewServer creates a Server backed by the given Checker. logs may be nil, in
 // which case log events are never sent to WatchEvents streams. configPath is
 // used by CheckConfig to reload and validate the configuration file on demand.
-// The provided context controls the lifetime of streaming RPCs: cancelling it
-// closes all active WatchEvents streams so that grpc.Server.GracefulStop can
-// complete.
-func NewServer(ctx context.Context, c *checker.Checker, logs *LogBroadcaster, configPath string) *Server {
-	return &Server{ctx: ctx, checker: c, logs: logs, configPath: configPath}
+// vppClient may be nil if VPP integration is disabled. The provided context
+// controls the lifetime of streaming RPCs: cancelling it closes all active
+// WatchEvents streams so that grpc.Server.GracefulStop can complete.
+func NewServer(ctx context.Context, c *checker.Checker, logs *LogBroadcaster, configPath string, vppClient *vpp.Client) *Server {
+	return &Server{ctx: ctx, checker: c, logs: logs, configPath: configPath, vppClient: vppClient}
 }
 
 // ListFrontends returns the names of all configured frontends.
@@ -257,6 +259,29 @@ func (s *Server) doReloadConfig() *ReloadConfigResponse {
 	return &ReloadConfigResponse{Ok: true}
 }
 
+// GetVPPInfo returns VPP version and runtime information.
+func (s *Server) GetVPPInfo(_ context.Context, _ *GetVPPInfoRequest) (*VPPInfo, error) {
+	if s.vppClient == nil {
+		return nil, status.Error(codes.Unavailable, "VPP integration is disabled")
+	}
+	info, err := s.vppClient.GetInfo()
+	if err != nil {
+		return nil, status.Errorf(codes.Unavailable, "%v", err)
+	}
+	var boottimeNs int64
+	if !info.BootTime.IsZero() {
+		boottimeNs = info.BootTime.UnixNano()
+	}
+	return &VPPInfo{
+		Version:        info.Version,
+		BuildDate:      info.BuildDate,
+		BuildDirectory: info.BuildDirectory,
+		Pid:            info.PID,
+		BoottimeNs:     boottimeNs,
+		ConnecttimeNs:  info.ConnectedSince.UnixNano(),
+	}, nil
+}
+
 // ---- conversion helpers ----------------------------------------------------
 
 func frontendToProto(name string, fe config.Frontend) *FrontendInfo {