export: keep the peer reading while the next batch is dressed

An exporter following the table took up to 65,536 changes at a time,
dressed them all, and only then sent them; and before each batch it
slept the 50 ms that lets a run of changes be gathered, whether or not
the table was already far ahead of it. The session's queue holds about
a thousand UPDATEs, so once the batch went out the exporter waited on
the peer reading it, and while the next batch was dressed the peer had
nothing to read. A profile of two routers exchanging their tables,
sampled in 25 ms windows, showed the two stages taking turns: the
exporters at two cores while the sessions were at none, then the other
way round, with a gap of 50 to 75 ms at the start of each cycle for the
sleep. Each task ran at about half a core.

A batch is now 4,096 changes, so that what one batch sends is being
read by the peer while the next is being dressed, and the gathering
sleep is skipped while the reader is behind the table: a run is in hand
already. A reader says whether its last poll caught up.

Testing: the server tests, the integration tests, the kernel tests and
every lab phase pass. On the mesh benchmark in the Linux VM, three
rounds each alternating with the previous commit, the larger table
(4.9 million routes) is across in 5.6 s instead of 6.7 s and the
smaller (2.5 million) in 4.4 s instead of 6.4 s; the whole exchange
takes 7.3 s of wall-clock time instead of 8.6 s, and 18.9 s of CPU time
instead of 20.5 s.

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

crates/rib/src/table/journal.rs

@@ -706,6 +706,11 @@ impl Reader {
         Progress { events, caught_up }
     }
 
+    /// Whether the last [`Reader::poll`] read everything there was.
+    pub fn caught_up(&self) -> bool {
+        self.caught_up
+    }
+
     /// Waits until there may be something to read. Returns at once if the
     /// last [`Reader::poll`] did not catch up.
     pub async fn changed(&mut self) {