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).

proto/maglev.proto

@@ -20,6 +20,7 @@ service Maglev {
   rpc WatchEvents(WatchRequest) returns (stream Event);
   rpc CheckConfig(CheckConfigRequest) returns (CheckConfigResponse);
   rpc ReloadConfig(ReloadConfigRequest) returns (ReloadConfigResponse);
+  rpc GetVPPInfo(GetVPPInfoRequest) returns (VPPInfo);
 }
 
 // ---- requests ---------------------------------------------------------------
@@ -63,6 +64,17 @@ message ReloadConfigResponse {
   string reload_error = 4;   // set when config is valid but the reload itself failed
 }
 
+message GetVPPInfoRequest {}
+
+message VPPInfo {
+  string version = 1;
+  string build_date = 2;
+  string build_directory = 3;
+  uint32 pid = 4;
+  int64 boottime_ns = 5;    // unix timestamp (ns) when VPP started (from /sys/boottime)
+  int64 connecttime_ns = 6; // unix timestamp (ns) when maglevd connected to VPP
+}
+
 message SetWeightRequest {
   string frontend = 1;
   string pool = 2;