Fix the third review of the origin changes

The prune test gains a third prefix with more than 500 rows newer than 72
hours, and checks every newer row survives and every older one is deleted, in
a helper shared by the three prefixes. The prefix page test uses an origin AS
the routing table does not carry. README's limit list says the routing table
drops the oldest waiting origin changes past 100,000, and the prefix page
entry says the page shows at most the newest 5,000. The tests read 5,000 and
100,000 from named limits instead of bare numbers, so the routing table's
waiting cap is now exported for them.

Model: deepseek-v4-pro

README.md

@@ -145,7 +145,7 @@ and origin AS can have two kept routes, one or none.
   prefix's traffic to. RouteWatch records the peer itself as the next hop. The
   page also lists the prefix's origin changes, newest first. An origin change is
   an AS starting to announce the prefix, when its first peer carries it, or
-  stopping, when its last peer no longer does. It shows at most the newest
+  stopping, when its last peer no longer does. The page shows at most the newest
   5,000. Each row shows:
     - the time
     - announced or withdrawn
@@ -577,6 +577,9 @@ What happens at each limit:
   and after the held-back messages of every peer whose session went down before
   it. It holds at most 150,000 such messages, about 150 MiB. At that number it
   takes nothing more off its queue until it has applied one of them.
+- Waiting origin changes: the routing table keeps at most 100,000 origin changes
+  waiting to be written. Past that, it drops the oldest, and the prefix handler
+  logs how many. The prefix page never shows a dropped origin change.
 
 To replace the image's Go soft limit, set `GOMEMLIMIT` in the environment, for
 example `-e GOMEMLIMIT=3GiB`. A limit close to the routing table's size makes