rib: a short row grows by doubling

A prefix's row of paths is given room for as many paths as there are
peers, and from there grows by a quarter at a time, one path at a time
while it is short. A peer sending every path it has (ADD-PATH) gives a
prefix several paths from one peer, so a row goes past the peers' count
from the first few routes and was then reallocated for each path.

A row shorter than the first allocation's room doubles instead, and
grows by a quarter only from there. Measured on the mesh benchmark, the
reallocations in the table's insert path fall by half; the memory held
is as before, as the rows that reach the quarter-growth are the ones
that were already allocated for it.

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

crates/rib/src/table/mod.rs

@@ -873,6 +873,11 @@ fn push_tight<T>(vec: &mut Vec<T>, value: T, expected: usize) {
         let first = expected.min(FIRST_ROOM);
         let more = if vec.capacity() < first {
             first - vec.len()
+        } else if vec.capacity() < FIRST_ROOM {
+            // A peer that sends several routes for a prefix (ADD-PATH)
+            // takes a row past the peers' count: doubled while small,
+            // and from there by a quarter.
+            vec.capacity()
         } else {
             (vec.len() / 4).max(1)
         };