Make the routewatch tests pass on a loaded host (closes #213)

Five prefix handler tests gave HandleMessage two seconds to apply a full
batch of 25,000 prefixes, which under the race detector on a loaded host
takes longer. A test now waits up to stepTimeout, 15 seconds, for each step
it expects.

The slowest tests filled production limits, such as the peering handler's
500,000 paths. The peering handler now takes its path limit, and the prefix
handler its limit on one write of the table's changes, as they take their
timer interval. Those tests give small limits, and the tests of the maximum
batch size lower it to one batch. This also fixes
#215.

The test helpers bound every wait, Stop included, so a test that hangs in
one fails with its own message.

Model: opus-5-5

TODO.md

@@ -35,6 +35,12 @@ tested.
 
 # Completed Steps
 
+- 2026-10-27: the tests of `internal/routewatch` no longer fail on a loaded
+  host. Five of them gave `HandleMessage` two seconds to apply a full prefix
+  batch, which under the race detector can take longer. A test now waits up to
+  15 seconds for each step it expects. The tests of the handlers' limits give
+  the handlers small ones, so the package no longer runs past its 90-second
+  limit (closes #213, #215)
 - 2026-10-27: the server works out the home page's cards every minute until it
   stops. It used to work them out only once, at startup: the end of the context
   that `Start` is given stopped the refreshes (closes #211)