Fix the second review of the origin changes

Stop now counts the origin changes inside a running write, in a new
writingOriginChanges field, and adds them to the count it logs. The prune test
checks which rows survive, including a prefix with rows on both sides of 72
hours. The prefix page tests cover 5,000 and 5,001 rows and a row's time and
peer count. New tests cover the dropped-rows warning and the dropped count's
reset. The prune floor is named minOriginChangesPerPrefix. The README, TODO,
package doc and doc comments are corrected to match.

Model: deepseek-v4-pro

README.md

@@ -143,13 +143,15 @@ and origin AS can have two kept routes, one or none.
   its origin ASes and every peer that carries it. Only the kept routes show an
   AS path, a next hop and a time. The next hop is the router the peer sends the
   prefix's traffic to. RouteWatch records the peer itself as the next hop. The
-  page also lists the prefix's origin changes, newest first. It shows at most
-  the newest 5,000. The peer count counts the peers of every origin AS that
-  carries the prefix. Each row shows:
+  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
+  5,000. Each row shows:
     - the time
     - announced or withdrawn
     - the origin AS
-    - the peer count
+    - the peer count: how many peers carry the prefix just after the change,
+      with any origin AS
 - `/prefixlength/{length}`: up to 500 IPv4 prefixes with that mask length,
   picked at random. Each shows its origin AS and the age of its newest kept
   route. When peers give a prefix different origin ASes, the page shows the one
@@ -765,8 +767,8 @@ What a stop can lose:
   changed pairs. A full-table peer is a peer that carries the whole routing
   table. Its session going down changes about a million pairs, which take about
   a minute to write. A stop in that minute leaves the rest unwritten.
-- The same last write takes at most 20,000 of the routing table's origin
-  changes. Origin changes past them are lost.
+- The prefix handler's last write of origin changes takes at most 20,000. The
+  rest are lost.
 - A withdrawal, or a session going down, that is lost this way leaves its routes
   in the `routes` table. The next start reads them back. They stay until the
   peer sends another update for the prefix, or its session goes down again.