Add WatchEvents, enable/disable/weight RPCs, and config check

gRPC / proto
- Rename WatchBackendEvents → WatchEvents; return a stream of Event
  oneof (LogEvent, BackendEvent, FrontendEvent) with optional filter
  flags (log, log_level, backend, frontend)
- Add EnableBackend, DisableBackend, SetFrontendPoolBackendWeight RPCs
- Rename PauseResumeRequest → BackendRequest
- Add CheckConfig RPC returning ok/parse_error/semantic_error

maglevd
- Route slog through a LogBroadcaster (slog.Handler) so WatchEvents
  subscribers can receive structured log records independently of the
  daemon's own --log-level
- Add --reflection flag (default true) to toggle gRPC server reflection
- Add --check flag: validates config file and exits 0/1/2
- SIGHUP: use config.Check before applying reload; log parse vs semantic
  error separately; refuse reload on any error
- Rename default config path /etc/maglev → /etc/vpp-maglev

maglevc
- Add 'watch events [num <n>] [log [level <level>]] [backend] [frontend]'
  command; prints compact protojson, stops on any keypress or Ctrl-C;
  uses cbreak mode (not raw) so output post-processing is preserved
- Add 'set backend <name> enable|disable'
- Add 'set frontend <name> pool <pool> backend <name> weight <0-100>'
- Add 'config check' command

Debian packaging
- Rename service unit to vpp-maglevd.service
- Rename conffiles to /etc/default/vpp-maglev and /etc/vpp-maglev/
- Create maglevd system user/group in postinst; add to vpp group if present
- Add postrm; add adduser to Depends

cmd/maglevd/main.go

@@ -13,6 +13,7 @@ import (
 	"syscall"
 
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/reflection"
 
 	buildinfo "git.ipng.ch/ipng/vpp-maglev/cmd"
 	"git.ipng.ch/ipng/vpp-maglev/internal/checker"
@@ -30,7 +31,9 @@ func main() {
 func run() error {
 	// ---- flags / env --------------------------------------------------------
 	printVersion := flag.Bool("version", false, "print version and exit")
-	configPath := stringFlag("config", "/etc/maglev/frontend.yaml", "MAGLEV_CONFIG", "path to frontend.yaml")
+	checkOnly := flag.Bool("check", false, "check config file and exit (0=ok, 1=parse error, 2=semantic error)")
+	enableReflection := flag.Bool("reflection", true, "enable gRPC server reflection (for grpcurl)")
+	configPath := stringFlag("config", "/etc/vpp-maglev/maglev.yaml", "MAGLEV_CONFIG", "path to maglev.yaml")
 	grpcAddr := stringFlag("grpc-addr", ":9090", "MAGLEV_GRPC_ADDR", "gRPC listen address")
 	logLevel := stringFlag("log-level", "info", "MAGLEV_LOG_LEVEL", "log level (debug|info|warn|error)")
 	flag.Parse()
@@ -41,12 +44,28 @@ func run() error {
 		return nil
 	}
 
+	if *checkOnly {
+		_, result := config.Check(*configPath)
+		if result.OK() {
+			fmt.Printf("config ok: %s\n", *configPath)
+			return nil
+		}
+		if result.ParseError != "" {
+			fmt.Fprintf(os.Stderr, "parse error: %s\n", result.ParseError)
+			os.Exit(1)
+		}
+		fmt.Fprintf(os.Stderr, "semantic error: %s\n", result.SemanticError)
+		os.Exit(2)
+	}
+
 	// ---- logging ------------------------------------------------------------
 	var level slog.Level
 	if err := level.UnmarshalText([]byte(*logLevel)); err != nil {
 		return fmt.Errorf("invalid log level %q: %w", *logLevel, err)
 	}
-	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: level})))
+	jsonHandler := slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: level})
+	logBroadcaster := grpcapi.NewLogBroadcaster(jsonHandler)
+	slog.SetDefault(slog.New(logBroadcaster))
 	slog.Info("starting", "version", buildinfo.Version(), "commit", buildinfo.Commit(), "date", buildinfo.Date())
 
 	// ---- config -------------------------------------------------------------
@@ -74,8 +93,11 @@ func run() error {
 		return fmt.Errorf("listen %s: %w", *grpcAddr, err)
 	}
 	srv := grpc.NewServer()
-	grpcapi.RegisterMaglevServer(srv, grpcapi.NewServer(ctx, chkr))
-	slog.Info("grpc-listening", "addr", *grpcAddr)
+	grpcapi.RegisterMaglevServer(srv, grpcapi.NewServer(ctx, chkr, logBroadcaster, *configPath))
+	if *enableReflection {
+		reflection.Register(srv)
+	}
+	slog.Info("grpc-listening", "addr", *grpcAddr, "reflection", *enableReflection)
 
 	go func() {
 		if err := srv.Serve(lis); err != nil {
@@ -91,9 +113,13 @@ func run() error {
 		switch sig {
 		case syscall.SIGHUP:
 			slog.Info("config-reload-start")
-			newCfg, err := config.Load(*configPath)
-			if err != nil {
-				slog.Error("config-reload-error", "err", err)
+			newCfg, result := config.Check(*configPath)
+			if !result.OK() {
+				if result.ParseError != "" {
+					slog.Error("config-check-failed", "type", "parse", "err", result.ParseError)
+				} else {
+					slog.Error("config-check-failed", "type", "semantic", "err", result.SemanticError)
+				}
 				continue
 			}
 			if err := chkr.Reload(ctx, newCfg); err != nil {