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).

go.mod

@@ -4,7 +4,9 @@ go 1.25.0
 
 require (
 	github.com/chzyer/readline v1.5.1
+	github.com/prometheus/client_golang v1.23.2
 	github.com/vishvananda/netns v0.0.5
+	go.fd.io/govpp v0.12.0
 	golang.org/x/net v0.52.0
 	golang.org/x/sys v0.43.0
 	google.golang.org/grpc v1.80.0
@@ -15,11 +17,15 @@ require (
 require (
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/fsnotify/fsnotify v1.9.0 // indirect
+	github.com/ftrvxmtrx/fd v0.0.0-20150925145434-c6d800382fff // indirect
+	github.com/kr/text v0.2.0 // indirect
+	github.com/lunixbochs/struc v0.0.0-20200521075829-a4cb8d33dbbe // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
-	github.com/prometheus/client_golang v1.23.2 // indirect
 	github.com/prometheus/client_model v0.6.2 // indirect
 	github.com/prometheus/common v0.66.1 // indirect
 	github.com/prometheus/procfs v0.16.1 // indirect
+	github.com/sirupsen/logrus v1.9.3 // indirect
 	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	golang.org/x/text v0.35.0 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20260120221211-b8f7ae30c516 // indirect