Page through the received routes, on the status page and in the API

/api/routes/received now returns one page: {routes, next_after,
prev_before}. Pages are in prefix order and begin at a prefix (after= or
before=), not at a count, so a table that changes between two pages does
not make them overlap or leave a gap. A page holds whole prefixes, about
`limit` routes' worth.

The table keeps prefixes in no order, so a page is found by looking at
every prefix once and keeping the nearest few; the routes of the others
are not touched (RibTable::for_each_prefix).

The status page shows about 100 routes with first / previous / next
links; the place is kept in the page's address with the lookup.

The response of /api/routes/received was an array before: this changes
its shape.

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

crates/integration-tests/tests/debugging.rs

@@ -42,8 +42,8 @@ async fn routes(client: &reqwest::Client, base: &str, query: &str) -> Result<Vec
         .send()
         .await?
         .error_for_status()?;
-    let routes: Value = reply.json().await?;
-    Ok(routes
+    let page: Value = reply.json().await?;
+    Ok(page["routes"]
         .as_array()
         .unwrap()
         .iter()
@@ -131,7 +131,9 @@ async fn test_routes_can_be_looked_up() -> Result<()> {
         ]
     );
     assert_eq!(routes(&client, &base, "").await?.len(), 5);
-    assert_eq!(routes(&client, &base, "limit=2").await?.len(), 2);
+    // A page holds whole prefixes: at most the limit, unless one prefix
+    // alone has more.
+    assert!((1..=2).contains(&routes(&client, &base, "limit=2").await?.len()));
 
     // One peer's routes, by its address; a peer that is not there has none.
     assert_eq!(routes(&client, &base, "peer=127.0.0.1").await?.len(), 5);