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

TODO.md

@@ -60,12 +60,15 @@ problem the run finds.
   The routing table records each change, and the prefix handler writes them to a
   new `origin_changes` table. The database maintainer prunes each prefix to its
   newest 500 rows or its rows from the last 72 hours, whichever keeps more. The
-  page shows at most the newest 5,000 (closes #161). The peer count counts the
-  peers of every origin AS. Each row shows:
+  page shows at most the newest 5,000. Each row shows:
     - the time
     - announced or withdrawn
     - the origin AS
     - the peer count
+
+    A row's peer count is how many peers carry the prefix just after the change,
+    with any origin AS (closes #161).
+
 - 2026-11-03: the streamer drops an update for a handler only when that
   handler's queue is full. It used to drop a random share of the updates once
   the queue was half full. The prefix handler's queue now holds 100,000