Install best routes in the Linux kernel over netlink

New crate bgp-fib. It follows the routing table's best routes and keeps
the kernel's routing table the same, and learns the machine's interfaces
and addresses from the kernel, which is what says whether a next hop can
be reached.

- netlink: the rtnetlink messages used (routes, addresses, links, errors),
  as bytes and back. No I/O; property tested and fuzzed.
- interfaces: what is directly reachable, and from which interface.
- plan: what to ask the kernel for, given the best routes and what the
  kernel has. No I/O; tested against a model kernel.
- kernel (Linux only): the sockets and the task. Requests go in batches;
  routes of ours found at startup are kept if still right and removed if
  not; a change of interfaces has every route worked out again; a clean
  stop removes our routes.

Only routes marked proto bgp with the configured table and metric are
ever touched. A best route is installed if its next hop is a neighbor;
next hops further away are not resolved through other routes yet, and
such prefixes are counted and shown instead.

Configured with `kernel { table metric }`; off by default. The daemon
needs CAP_NET_ADMIN and stops if the kernel refuses it. On shutdown it
closes sessions and then removes its routes.

/api/kernel and the status page show installed, unresolved and refused
counts, the last error and the interfaces; five bgp_kernel_* metrics.

Testing: scripts/kernel-tests.sh runs, in network namespaces of their
own and with only the capabilities the daemon needs, the installer
against `ip route`, the daemon with a peer, and two daemons that route
for each other with pings across over IPv4 and IPv6.
scripts/linux-vm.sh runs that, or anything else, in a Lima VM from
macOS. CI runs the kernel tests and fuzzes the netlink decoder.

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

crates/rib/src/table/mod.rs

@@ -352,6 +352,13 @@ impl RibTable {
     /// with it, and may then be given to another. Whoever was given it must
     /// not use it after that: the table cannot tell a late announcement
     /// from one of the number's next owner.
+    /// What is known of the peer with this number, if it is in use. Not to
+    /// be called from a visitor that is given routes with a shard held.
+    pub fn peer(&self, index: PeerIndex) -> Option<PeerInfo> {
+        let peers = self.peers.lock().unwrap_or_else(PoisonError::into_inner);
+        peers.get(index.as_usize()).copied().flatten()
+    }
+
     pub fn add_peer(&self, info: PeerInfo) -> Result<PeerIndex, TooManyPeers> {
         let mut peers = self.peers.lock().unwrap_or_else(PoisonError::into_inner);
         let index = match peers.iter().position(Option::is_none) {