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

docs/user-guide.md

@@ -10,9 +10,11 @@ inspection and control.
 
 | Flag | Environment variable | Default | Description |
 |---|---|---|---|
-| `--config` | `MAGLEV_CONFIG` | `/etc/maglev/maglev.yaml` | Path to the YAML configuration file. |
+| `--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. |
 | `--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. |
 | `--version` | — | — | Print version, commit hash, and build date, then exit. |
 
 Flags take precedence over environment variables. Both are optional; defaults
@@ -22,7 +24,7 @@ are used for anything not set.
 
 | Signal | Effect |
 |---|---|
-| `SIGHUP` | Reload the configuration file. New backends are started, removed backends are stopped, backends whose health-check config is unchanged continue probing without interruption. |
+| `SIGHUP` | Reload the configuration file. The file is checked before applying; if there is a parse or semantic error the reload is aborted and the error is logged (the daemon continues running with its current config). New backends are started, removed backends are stopped, backends whose health-check config is unchanged continue probing without interruption. |
 | `SIGTERM` / `SIGINT` | Graceful shutdown. Active gRPC streams are closed, the server drains, then the process exits. |
 
 ### Capabilities
@@ -83,6 +85,36 @@ show healthcheck <name>          Show full health-check configuration.
 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.
+set frontend <name> pool <pool> backend <name> weight <0-100>
+                                 Set the weight of a backend within a pool. Weight 0 keeps
+                                 the backend in the pool but assigns it no traffic.
+                                 Takes effect immediately without reloading configuration.
+
+set backend <name> disable       Stop probing entirely and remove the backend from rotation.
+                                 The backend remains visible (state: removed) and can be
+                                 re-enabled without reloading configuration.
+set backend <name> enable        Re-enable a disabled backend. A fresh probe goroutine is
+                                 started and the backend re-enters unknown state.
+
+watch events                     Stream all events (log, backend transitions, frontend)
+  [num <n>]                      Stop after receiving n events.
+  [log [level <level>]]          Include log events. level is debug|info|warn|error
+                                 (default: info). Omitting log/backend/frontend enables all.
+  [backend]                      Include backend transition events.
+  [frontend]                     Include frontend events (reserved for future use).
+
+                                 Each event is printed as compact JSON on its own line.
+                                 Press any key or Ctrl-C to stop. Examples:
+
+                                   watch events
+                                   watch events num 20
+                                   watch events log level debug
+                                   watch events backend num 100
+                                   watch events log level debug backend
+
+config check                     Ask maglevd to read and validate its current config file.
+                                 Prints "config ok" on success, or the error (parse or
+                                 semantic) returned by the daemon.
 
 quit / exit                      Leave the interactive shell.
 ```