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/rib/src/table/mod.rs

@@ -215,6 +215,23 @@ impl Row {
     }
 }
 
+/// A prefix in the table with its routes, looked at in place.
+#[derive(Clone, Copy)]
+pub struct PrefixRef<'a> {
+    row: &'a Row,
+}
+
+impl<'a> PrefixRef<'a> {
+    pub fn prefix(&self) -> &'a IpPrefix {
+        &self.row.prefix
+    }
+
+    /// The routes there are for the prefix: at least one.
+    pub fn routes(&self) -> impl Iterator<Item = RouteRef<'a>> + use<'a> {
+        self.row.routes()
+    }
+}
+
 /// A route in the table, looked at in place.
 #[derive(Debug, Clone, Copy)]
 pub struct RouteRef<'a> {
@@ -556,6 +573,21 @@ impl RibTable {
         routes
     }
 
+    /// Calls `visit` with every prefix that has routes, in no particular
+    /// order. As [`RibTable::for_each_best`]: the table is not stopped,
+    /// and `visit` must not call into it.
+    ///
+    /// The prefix's routes are there to be looked at, and cost nothing if
+    /// they are not: this is how to pick a few prefixes out of a table
+    /// with a great many routes for each.
+    pub fn for_each_prefix(&self, mut visit: impl FnMut(PrefixRef<'_>)) {
+        for shard in &self.shards {
+            for row in &read(shard).rows {
+                visit(PrefixRef { row });
+            }
+        }
+    }
+
     /// Calls `visit` with every route in the table. As
     /// [`RibTable::for_each_best`]: the table is not stopped, and `visit`
     /// must not call into it.