Code health after review: docs, metrics, structure, one more fuzz target

What the code-health review found, and what a fuzz target written for it
found in its first seconds.

- A set of attributes with the same attribute twice is refused by the
  routing table. Its readers (the selection key, the path, the next hop)
  could each take a different one of the two. The decoder never lets one
  through; the new fuzz target `attrs` found that the table would.
- bgp_peer_up{peer}, to alert on a peer going away, and bgp_build_info.
  Every metric is there from the first scrape. Alerts for a peer down,
  hijack attempts and kernel errors are in the README.
- `bgp --check config` validates a configuration without starting, and
  `--version` says which build it is. CI runs the first on the example
  configuration, which a test now also parses with its commented fields
  put in; the example had to learn about the next hop check.
- Path attributes have a module of their own, packet::attributes,
  re-exported from packet::message as before.
- policy::From, which shadowed the trait, is RouteSource. The server's
  connection, dialer and fsm modules are private. Logging arguments live
  in the binary; the server library no longer depends on clap.
- One is_unicast_link_local (the standard library's) instead of two of
  our own; SelectionKey::origin(); one place where an Import is made.
- READMEs say what the daemon does today rather than what it did not do
  a year ago. Documentation links are checked in CI. Unused dependencies
  and bgp_web::serve are gone; the Docker build is --locked and does not
  ship the fuzz corpus.

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

crates/server/src/session.rs

@@ -23,7 +23,7 @@ use crate::address::{Endpoint, PeerAddress, socket_text};
 use crate::collision::Direction;
 use crate::family::FamilySet;
 use crate::fsm::Negotiated;
-use crate::metrics::SESSIONS_ESTABLISHED;
+use crate::metrics::{PEER_UP, SESSIONS_ESTABLISHED};
 use crate::policy::PeerRole;
 use crate::rpki::Subject;
 use crate::sync::IgnorePoisonRw;
@@ -262,6 +262,9 @@ impl SessionRegistry {
             .iter()
             .map(|p| (p.name.clone(), DialState::Idle))
             .collect();
+        for peer in &configured {
+            PEER_UP.with_label_values(&[&peer.name]).set(0);
+        }
         Self {
             inner: Arc::new(RwLock::new(HashMap::new())),
             configured: Arc::new(configured),
@@ -306,6 +309,9 @@ impl SessionRegistry {
         let peer = handle.peer;
         self.inner.writing().insert(peer, handle.clone());
         SESSIONS_ESTABLISHED.inc();
+        if let Some(name) = &handle.name {
+            PEER_UP.with_label_values(&[name]).set(1);
+        }
         let _ = self.events.send(SessionEvent::Up(handle));
         SessionGuard {
             registry: self.clone(),
@@ -362,8 +368,11 @@ impl SessionRegistry {
 
     fn remove(&self, peer: &PeerId) {
         let removed = self.inner.writing().remove(peer);
-        if removed.is_some() {
+        if let Some(handle) = &removed {
             SESSIONS_ESTABLISHED.dec();
+            if let Some(name) = &handle.name {
+                PEER_UP.with_label_values(&[name]).set(0);
+            }
             let _ = self.events.send(SessionEvent::Down(*peer));
         }
     }