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/config-guide.md

@@ -11,7 +11,7 @@ in two stages:
     ensuring that every backend referenced by a frontend exists, that address families are
     consistent within a frontend, and that IP source addresses are the correct family.
 
-If you want to get started quickly, take a look at the [example config](#example).
+If you want to get started quickly, take a look at the [[example config](../debian/mavleg.yaml)].
 
 ## Basic structure
 
@@ -277,109 +277,7 @@ frontends:
 
 ---
 
-## Example
+For a detailed description of the health state machine, probe intervals, and all transition events,
+see [[healthchecks.md](healthchecks.md)]. For a user guide on how to use the maglev daemon and client,
+see the [[user-guide.md](user-guide.md)].
 
-A complete configuration tying all sections together:
-
-```yaml
-maglev:
-  healthchecker:
-    transition-history: 5
-    netns: dataplane
-
-  healthchecks:
-    nginx:
-      type: http
-      port: 80
-      params:
-        path: /healthz
-        host: nginx.example.com
-        response-code: "200"
-      interval: 2s
-      fast-interval: 500ms
-      down-interval: 30s
-      timeout: 3s
-      rise: 2
-      fall: 3
-
-    dovecot:
-      type: tcp
-      port: 993
-      params:
-        ssl: true
-        server-name: imaps.example.com
-      interval: 5s
-      fast-interval: 1s
-      down-interval: 30s
-      timeout: 3s
-      rise: 2
-      fall: 3
-
-    ping6:
-      type: icmp
-      probe-ipv6-src: 2001:db8:probe::1
-      interval: 2s
-      timeout: 1s
-
-  backends:
-    nginx0-ams:
-      address: 198.51.100.10
-      healthcheck: nginx
-    nginx0-lon:
-      address: 198.51.100.11
-      healthcheck: nginx
-    nginx0-fra:
-      address: 198.51.100.12
-      healthcheck: nginx
-    maildrop0-ams:
-      address: 2001:db8:1::10
-      healthcheck: dovecot
-    maildrop0-lon:
-      address: 2001:db8:1::11
-      healthcheck: dovecot
-
-  frontends:
-    nginx-http:
-      description: "HTTP VIP with fallback"
-      address: 198.51.100.1
-      protocol: tcp
-      port: 80
-      pools:
-        - name: primary
-          backends:
-            nginx0-ams: { weight: 10 }
-            nginx0-lon: {}
-        - name: fallback
-          backends:
-            nginx0-fra: {}
-
-    nginx-https:
-      description: "HTTPS VIP — same backends, different port"
-      address: 198.51.100.1
-      protocol: tcp
-      port: 443
-      pools:
-        - name: primary
-          backends:
-            nginx0-ams: { weight: 10 }
-            nginx0-lon: {}
-        - name: fallback
-          backends:
-            nginx0-fra: {}
-
-    maildrop-imaps:
-      description: "IMAPS VIP"
-      address: 2001:db8::1
-      protocol: tcp
-      port: 993
-      pools:
-        - name: primary
-          backends:
-            maildrop0-ams: {}
-            maildrop0-lon: {}
-```
-
----
-
-For a detailed description of the health state machine, probe intervals, and all
-transition events, see [healthchecks.md](healthchecks.md).