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/maglev_grpc.pb.go

@@ -33,6 +33,7 @@ const (
 	Maglev_WatchEvents_FullMethodName                  = "/maglev.Maglev/WatchEvents"
 	Maglev_CheckConfig_FullMethodName                  = "/maglev.Maglev/CheckConfig"
 	Maglev_ReloadConfig_FullMethodName                 = "/maglev.Maglev/ReloadConfig"
+	Maglev_GetVPPInfo_FullMethodName                   = "/maglev.Maglev/GetVPPInfo"
 )
 
 // MaglevClient is the client API for Maglev service.
@@ -55,6 +56,7 @@ type MaglevClient interface {
 	WatchEvents(ctx context.Context, in *WatchRequest, opts ...grpc.CallOption) (grpc.ServerStreamingClient[Event], error)
 	CheckConfig(ctx context.Context, in *CheckConfigRequest, opts ...grpc.CallOption) (*CheckConfigResponse, error)
 	ReloadConfig(ctx context.Context, in *ReloadConfigRequest, opts ...grpc.CallOption) (*ReloadConfigResponse, error)
+	GetVPPInfo(ctx context.Context, in *GetVPPInfoRequest, opts ...grpc.CallOption) (*VPPInfo, error)
 }
 
 type maglevClient struct {
@@ -214,6 +216,16 @@ func (c *maglevClient) ReloadConfig(ctx context.Context, in *ReloadConfigRequest
 	return out, nil
 }
 
+func (c *maglevClient) GetVPPInfo(ctx context.Context, in *GetVPPInfoRequest, opts ...grpc.CallOption) (*VPPInfo, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(VPPInfo)
+	err := c.cc.Invoke(ctx, Maglev_GetVPPInfo_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
 // MaglevServer is the server API for Maglev service.
 // All implementations must embed UnimplementedMaglevServer
 // for forward compatibility.
@@ -234,6 +246,7 @@ type MaglevServer interface {
 	WatchEvents(*WatchRequest, grpc.ServerStreamingServer[Event]) error
 	CheckConfig(context.Context, *CheckConfigRequest) (*CheckConfigResponse, error)
 	ReloadConfig(context.Context, *ReloadConfigRequest) (*ReloadConfigResponse, error)
+	GetVPPInfo(context.Context, *GetVPPInfoRequest) (*VPPInfo, error)
 	mustEmbedUnimplementedMaglevServer()
 }
 
@@ -286,6 +299,9 @@ func (UnimplementedMaglevServer) CheckConfig(context.Context, *CheckConfigReques
 func (UnimplementedMaglevServer) ReloadConfig(context.Context, *ReloadConfigRequest) (*ReloadConfigResponse, error) {
 	return nil, status.Error(codes.Unimplemented, "method ReloadConfig not implemented")
 }
+func (UnimplementedMaglevServer) GetVPPInfo(context.Context, *GetVPPInfoRequest) (*VPPInfo, error) {
+	return nil, status.Error(codes.Unimplemented, "method GetVPPInfo not implemented")
+}
 func (UnimplementedMaglevServer) mustEmbedUnimplementedMaglevServer() {}
 func (UnimplementedMaglevServer) testEmbeddedByValue()                {}
 
@@ -552,6 +568,24 @@ func _Maglev_ReloadConfig_Handler(srv interface{}, ctx context.Context, dec func
 	return interceptor(ctx, in, info, handler)
 }
 
+func _Maglev_GetVPPInfo_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(GetVPPInfoRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(MaglevServer).GetVPPInfo(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: Maglev_GetVPPInfo_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(MaglevServer).GetVPPInfo(ctx, req.(*GetVPPInfoRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
 // Maglev_ServiceDesc is the grpc.ServiceDesc for Maglev service.
 // It's only intended for direct use with grpc.RegisterService,
 // and not to be introspected or modified (even as a copy)
@@ -611,6 +645,10 @@ var Maglev_ServiceDesc = grpc.ServiceDesc{
 			MethodName: "ReloadConfig",
 			Handler:    _Maglev_ReloadConfig_Handler,
 		},
+		{
+			MethodName: "GetVPPInfo",
+			Handler:    _Maglev_GetVPPInfo_Handler,
+		},
 	},
 	Streams: []grpc.StreamDesc{
 		{