Wait up to 30 seconds for each step the routewatch tests expect (closes #213)

Five prefix handler tests gave HandleMessage two seconds to apply a full
batch of 25,000 prefixes. Under the race detector on a loaded host that
takes longer, so they failed although nothing waited. Among them were
TestPrefixHandlerSkipsItsLastWriteAfterTheStopDeadline,
TestPrefixHandlerSkipsAFullBatchAfterTheStopDeadline and
TestPrefixHandlerHandlesMessagesWhileAWriteIsBlocked.

handleMessageTimeout is now 30 seconds, and every bound a test in the
package puts on a step it expects uses it. Only a step that never comes
reaches it. A test that hangs still fails with its own message before go
test's 90-second timeout.

Model: opus-5-5

TODO.md

@@ -35,6 +35,10 @@ 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
+  30 seconds for each step it expects (closes #213)
 - 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)