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).

docs/user-guide.md

@@ -13,6 +13,8 @@ inspection and control.
 | `--config` | `MAGLEV_CONFIG` | `/etc/vpp-maglev/maglev.yaml` | Path to the YAML configuration file. |
 | `--grpc-addr` | `MAGLEV_GRPC_ADDR` | `:9090` | TCP address on which the gRPC server listens. |
 | `--metrics-addr` | `MAGLEV_METRICS_ADDR` | `:9091` | TCP address for the Prometheus `/metrics` HTTP endpoint. Set to empty to disable. |
+| `--vpp-api-addr` | `MAGLEV_VPP_API_ADDR` | `/run/vpp/api.sock` | VPP binary API socket path. Set to empty to disable VPP integration. |
+| `--vpp-stats-addr` | `MAGLEV_VPP_STATS_ADDR` | `/run/vpp/stats.sock` | VPP stats socket path. |
 | `--log-level` | `MAGLEV_LOG_LEVEL` | `info` | Log verbosity: `debug`, `info`, `warn`, or `error`. |
 | `--check` | — | — | Read and validate the config file, then exit. Exits 0 if the config is valid, 1 on YAML parse error, 2 on semantic error. |
 | `--reflection` | — | `true` | Enable gRPC server reflection. Allows `grpcurl` to introspect the API without the `.proto` file. Set to `false` to disable. |
@@ -83,6 +85,10 @@ show backends [<name>]           Without name: list all backend names.
 show healthchecks [<name>]       Without name: list all health-check names.
                                  With name: show full health-check configuration.
 
+show vpp info                    Show VPP version, build date, PID, uptime, and when
+                                 maglevd connected. Returns an error if VPP is not
+                                 connected.
+
 set backend <name> pause         Suspend health checking for a backend, freezing its state.
 set backend <name> resume        Resume health checking; backend re-enters unknown state
                                  and is probed immediately.