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>

scripts/kernel-tests.sh

@@ -0,0 +1,38 @@
+#!/bin/sh
+# Runs the tests that need a real Linux kernel to change the routes of:
+# those of the bgp-fib crate that `cargo test` skips, and the end-to-end
+# test of two daemons that route for each other.
+#
+# They run in a network namespace made for the purpose, so the machine's
+# own interfaces and routes are out of their reach, and with the one
+# capability the daemon needs, CAP_NET_ADMIN, rather than as root.
+#
+# Run it on Linux, as a user who can sudo. On another system,
+# scripts/linux-vm.sh runs it in a virtual machine.
+set -eu
+
+cd "$(dirname "$0")/.."
+[ "$(uname -s)" = Linux ] || { echo "this needs Linux; see scripts/linux-vm.sh" >&2; exit 1; }
+
+# Built as the user, so that nothing in the target directory is root's.
+cargo build --locked -p bgp
+cargo test --locked -p bgp-fib --test kernel --no-run
+cargo test --locked -p bgp-integration-tests --test kernel --no-run
+
+namespace="ferrix-test-$$"
+sudo modprobe dummy 2>/dev/null || true
+sudo ip netns add "$namespace"
+trap 'sudo ip netns del "$namespace"' EXIT
+sudo ip netns exec "$namespace" ip link set lo up
+
+# In the namespace, as the user, with CAP_NET_ADMIN and nothing else.
+in_namespace() {
+    sudo ip netns exec "$namespace" \
+        setpriv --reuid "$(id -u)" --regid "$(id -g)" --init-groups \
+        --inh-caps +net_admin --ambient-caps +net_admin \
+        env PATH="$PATH" HOME="$HOME" CARGO_TARGET_DIR="${CARGO_TARGET_DIR:-}" "$@"
+}
+
+in_namespace cargo test --locked -p bgp-fib --test kernel -- --include-ignored --nocapture "$@"
+in_namespace cargo test --locked -p bgp-integration-tests --test kernel -- --include-ignored --nocapture "$@"
+scripts/two-routers.sh