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>

docs/RFCs.md

@@ -31,7 +31,9 @@ The base protocol specification.
 | §6.8 | Connection collision detection (BGP Identifier comparison, Cease / Connection Collision Resolution) | ✅ |
 | §8.1.1 | Optional `CollisionDetectEstablishedState` (`collision_detect_established_state`) | ✅ |
 | §6.2 | Bad BGP Identifier check (peer using our identifier, or 0.0.0.0) | ✅ |
-| §9 | Decision process / best-path selection | ❌ |
+| §9 | Decision process / best-path selection | ✅ |
+| §9.1.2 | Best routes installed for forwarding (Linux kernel, over netlink), where the next hop is directly connected | ✅ |
+| §9.1.2.1 | Routes with an unresolvable next hop excluded from selection; next hops resolved through other routes | ❌ (such a route can be chosen, and is then not installed) |
 | §9 | Adj-RIB-In (per-peer table of received routes) | ✅ |
 | §9 | Locally originated routes, advertised to all established peers | ✅ |
 | §9 | Loc-RIB / Adj-RIB-Out for routes learned from peers (re-advertisement) | ❌ |