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>

fuzz/fuzz_targets/attrs.rs

@@ -0,0 +1,31 @@
+//! The routing table's own reading of attribute bytes. A set is kept as
+//! the bytes it came in, and what selection and validation need (the
+//! path, the origin, the next hop) is read straight from them, by code
+//! other than the decoder's. Nothing a peer sends may make that panic,
+//! and what it reads must be what the decoder would have read.
+#![no_main]
+
+use bgp_rib::table::AttrStore;
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &[u8]| {
+    let store = AttrStore::new();
+    let Some(set) = store.intern_wire(data) else {
+        return;
+    };
+    let key = set.key();
+    let mut path = Vec::new();
+    let has_set = set.as_path(&mut path);
+    let _ = set.next_hop();
+    // The origin is the path's last AS, unless a set is in the way.
+    if let Some(origin) = key.origin() {
+        assert_eq!(path.last(), Some(&origin));
+    }
+    if !has_set {
+        assert_eq!(usize::from(key.as_path_len), path.len());
+    }
+    // The same bytes are the same set.
+    let again = store.intern_wire(data).expect("it was a set a moment ago");
+    assert!(std::sync::Arc::ptr_eq(&set, &again));
+    let _ = set.attributes();
+});