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

proto/maglev.proto

@@ -10,11 +10,15 @@ service Maglev {
   rpc GetFrontend(GetFrontendRequest) returns (FrontendInfo);
   rpc ListBackends(ListBackendsRequest) returns (ListBackendsResponse);
   rpc GetBackend(GetBackendRequest) returns (BackendInfo);
-  rpc PauseBackend(PauseResumeRequest) returns (BackendInfo);
-  rpc ResumeBackend(PauseResumeRequest) returns (BackendInfo);
+  rpc PauseBackend(BackendRequest) returns (BackendInfo);
+  rpc ResumeBackend(BackendRequest) returns (BackendInfo);
+  rpc EnableBackend(BackendRequest) returns (BackendInfo);
+  rpc DisableBackend(BackendRequest) returns (BackendInfo);
   rpc ListHealthChecks(ListHealthChecksRequest) returns (ListHealthChecksResponse);
   rpc GetHealthCheck(GetHealthCheckRequest) returns (HealthCheckInfo);
-  rpc WatchBackendEvents(WatchRequest) returns (stream BackendEvent);
+  rpc SetFrontendPoolBackendWeight(SetWeightRequest) returns (FrontendInfo);
+  rpc WatchEvents(WatchRequest) returns (stream Event);
+  rpc CheckConfig(CheckConfigRequest) returns (CheckConfigResponse);
 }
 
 // ---- requests ---------------------------------------------------------------
@@ -31,7 +35,7 @@ message GetBackendRequest {
   string name = 1;
 }
 
-message PauseResumeRequest {
+message BackendRequest {
   string name = 1;
 }
 
@@ -41,7 +45,29 @@ message GetHealthCheckRequest {
   string name = 1;
 }
 
-message WatchRequest {}
+message CheckConfigRequest {}
+
+message CheckConfigResponse {
+  bool ok = 1;
+  string parse_error = 2;    // set when YAML cannot be read or parsed
+  string semantic_error = 3; // set when YAML is valid but semantically incorrect
+}
+
+message SetWeightRequest {
+  string frontend = 1;
+  string pool = 2;
+  string backend = 3;
+  int32 weight = 4; // 0-100
+}
+
+// WatchRequest controls which event types are streamed. All fields default to
+// true (i.e. an empty request subscribes to everything at info level).
+message WatchRequest {
+  optional bool log = 1;       // include log events (default: true)
+  string log_level = 2;        // minimum log level: debug|info|warn|error (default: info)
+  optional bool backend = 3;   // include backend transition events (default: true)
+  optional bool frontend = 4;  // include frontend events (default: true)
+}
 
 // ---- responses --------------------------------------------------------------
 
@@ -123,7 +149,36 @@ message TransitionRecord {
   int64 at_unix_ns = 3;
 }
 
+// ---- event stream -----------------------------------------------------------
+
+// LogAttr is a single key/value attribute from a structured log record.
+message LogAttr {
+  string key = 1;
+  string value = 2;
+}
+
+// LogEvent carries a single structured log record.
+message LogEvent {
+  int64 at_unix_ns = 1;
+  string level = 2;
+  string msg = 3;
+  repeated LogAttr attrs = 4;
+}
+
+// BackendEvent is emitted on every backend state transition.
 message BackendEvent {
   string backend_name = 1;
   TransitionRecord transition = 2;
 }
+
+// FrontendEvent is reserved for future frontend-level events.
+message FrontendEvent {}
+
+// Event is the envelope returned by WatchEvents.
+message Event {
+  oneof event {
+    LogEvent log = 1;
+    BackendEvent backend = 2;
+    FrontendEvent frontend = 3;
+  }
+}