Server: one way to work on the table off-thread, and smaller tidying

Removing a peer's routes, sweeping its stale ones and sweeping the
attribute sets were each written out as their own spawn_blocking, and
each dealt with a failed task differently: the periodic sweep dropped a
panic on the floor, and a restarting peer whose kept routes could not be
removed was logged as "removed 0 routes". off_thread does it once, and
every caller logs a failure.

Also:
- the hard-reset wrapping of a NOTIFICATION is one function in the FSM;
- the two ways to close every session are one, and the registry has one
  leave() rather than leave() and shutdown();
- the dialer says why a session ended with SessionEnd::reason, and no
  longer turns a result into a result;
- as_path_prepend is parsed in one place, and the limits in error texts
  come from the constants rather than being written out;
- router_id and cluster_id are parsed by one function;
- the exporter builds its Egress in one place;
- the fields a session keeps only to hand to its handle are fewer:
  route_refresh is a bool, not a copy of the families;
- counting the stale routes removed cannot underflow;
- docs that had gone stale: the proto still said there was no route
  reflection, the advertiser's module doc said it announced only our own
  routes, from_server listed two of its five sources, and so on.

Tests for what had none: which session ends count as restarts, the Hard
Reset wrapping, the graceful restart settings, the dialer's jitter,
withdrawals in MP_UNREACH_NLRI, and AS_TRANS on a two-octet session.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/integration-tests/tests/announce.rs

@@ -393,7 +393,10 @@ async fn test_shutdown_sends_cease_to_every_peer() -> Result<()> {
     let mut b = established_peer_with_id(&server, Ipv4Addr::new(10, 0, 0, 2)).await?;
     wait_for_sessions(&server, 2).await?;
 
-    let left = server.registry.shutdown(Duration::from_secs(2)).await;
+    let left = server
+        .registry
+        .leave(bgp_server::restart::Leave::Stop, Duration::from_secs(2))
+        .await;
     assert_eq!(left, 0, "sessions still open after shutdown");
     assert!(server.registry.is_shutting_down());