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/maglevc/tree_test.go

@@ -28,6 +28,7 @@ func TestExpandPathsRoot(t *testing.T) {
 		"watch events",
 		"watch events <opt>",
 		"config check",
+		"show vpp info",
 		"config reload",
 		"quit",
 		"exit",
@@ -59,9 +60,9 @@ func TestExpandPathsShow(t *testing.T) {
 		}
 	}
 	// version, frontends, frontends <name>, backends, backends <name>,
-	// healthchecks, healthchecks <name> = 7 lines
-	if len(lines) != 7 {
-		t.Errorf("expected exactly 7 show subcommands, got %d", len(lines))
+	// healthchecks, healthchecks <name>, vpp info = 8 lines
+	if len(lines) != 8 {
+		t.Errorf("expected exactly 8 show subcommands, got %d", len(lines))
 	}
 }