Add an interoperability lab with FRR, BIRD and Routinator

scripts/interop/lab.sh runs the daemon against FRR, BIRD and Routinator,
each in a network namespace on one exchange LAN, and checks what every
side ends up with: sessions and routes over IPv4 and IPv6 with their
attributes, four-octet AS numbers, ADD-PATH, RFC 9234 roles and Only to
Customer, RTR version 2 and invalid routes, route refresh, session resets,
killed peers, a cut wire, the daemon stopped and killed, a table of a
million routes and a restart under it, role mismatch and the prefix limit.
All of it passes against FRR 10.5.1, BIRD 2.18 and Routinator 0.15.2.

scripts/interop/real-rpki.sh checks the RTR client against a Routinator
with the real RPKI: 713,258 IPv4 and 187,479 IPv6 VRPs and 3,379 ASPAs,
the same numbers on both sides.

What the lab found, fixed here:

- A daemon restarted with its routes in the kernel removed them all at
  once, its table being empty until the peers had sent theirs again, and
  then put them back: no forwarding meanwhile. Routes of an earlier run
  are now kept for `stale_seconds` (120 by default) and only those nobody
  announced again are removed. A restart under a million routes takes
  1.1 s to be in step, with nothing removed.
- FRR ends a session it loses to a collision with a state machine error
  without a subcode, which was shown as "error code 5 subcode 0".

CI gets a job that runs the lab.

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

crates/server/src/config.rs

@@ -184,6 +184,10 @@ impl Config {
                     table: or(kernel.table, defaults.table),
                     metric: or(kernel.metric, defaults.metric),
                     nexthop_objects: !kernel.without_nexthop_objects,
+                    stale_time: match kernel.stale_seconds {
+                        0 => defaults.stale_time,
+                        seconds => Duration::from_secs(seconds.into()),
+                    },
                 }
             }),
             rpki,
@@ -746,11 +750,13 @@ mod tests {
                 table: 254,
                 metric: 20,
                 nexthop_objects: true,
+                stale_time: Duration::from_secs(120),
             })
         );
         assert_eq!(
             parse(&format!(
-                "{BASE}kernel {{ table: 1000 metric: 150 without_nexthop_objects: true }}"
+                "{BASE}kernel {{ table: 1000 metric: 150 without_nexthop_objects: true \
+                 stale_seconds: 30 }}"
             ))
             .unwrap()
             .kernel,
@@ -758,6 +764,7 @@ mod tests {
                 table: 1000,
                 metric: 150,
                 nexthop_objects: false,
+                stale_time: Duration::from_secs(30),
             })
         );
     }