Policy scripts: the operator's own rules for routes, in Rhai

A new crate, bgp-policy, runs functions of a script on routes: on their
way in, after the general checks of a peer's import and before the
checks that can change (RPKI, next hop reachability); and on their way
out to a peer. A function is given the route and the peer, may refuse
the route, and may change its communities, large communities,
LOCAL_PREF and MED, and on the way out prepend our AS:

    fn from_peer(route, peer) {
        if route.has_community("65535:666") { return false; }
        route.add_community(LEARNED_FROM[peer.role]);
        route.local_pref = PREFERENCE[peer.role];
        true
    }

Configuration: policy { path | source, import_function, export_function }
and, for a peer, import { function } and export { function }. The script
is compiled at startup; `bgp --check` reads it and says which function
is whose. A function that fails on a route has refused it. Scripts have
no files, network or clock, and are stopped after 200,000 steps.

Route selection now goes by a LOCAL_PREF on a route from an external
peer: the decoder never lets a peer's own through, so one that is there
was set by policy.

What it costs was measured before it was built on (benches/policy.rs):
a call that tests a few communities and changes two takes 1 to 3
microseconds, some two hundred times what Rust would. So a function is
asked once for the routes of an UPDATE, which share their attributes,
unless it asks for the prefix. One peer sending 1.23 M routes over a
session: 0.37 s without a function, 0.80 s with one that marks and
ranks every route, 3.4 s if it also looks at the prefix.

Export applies to what is announced, which today is what the daemon
originates; learned routes are not passed on yet.

Tested by unit tests of the crate, sessions end to end, a fuzz target on
the example script (testdata/policy.rhai), and a lab phase in which a
script changes which of FRR's and BIRD's route is chosen and what each
of them is told.

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

README.md

@@ -16,7 +16,7 @@ It has not yet run in a production network.
 
 ### Not there yet
 
-- Announcing learned routes to other peers, and an export policy for it
+- Announcing learned routes to other peers (the export policy for it is there: scripts)
 - TCP-AO / TCP MD5 and GTSM
 - Graceful restart, BFD, changing the configuration without a restart
 - Next hops that are not directly connected
@@ -30,6 +30,7 @@ It has not yet run in a production network.
 | `crates/rib` | The routing table, best route selection, the journal of changes |
 | `crates/rpki` | The RTR client; origin validation and ASPA path verification |
 | `crates/fib` | The best routes in the Linux kernel, over netlink |
+| `crates/policy` | The operator's own rules for routes, as Rhai scripts |
 | `crates/server` | Sessions, the state machine, policy, configuration |
 | `crates/web` | The status page, HTTP API and metrics |
 | `bin/bgp` | The daemon |