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

internal/database/interface.go

@@ -1,6 +1,7 @@
 // Package database provides SQLite storage for BGP routing data: ASNs,
-// prefixes, announcements, peerings and BGP peers. Its routes table holds a
-// copy of the whole routing table that internal/routetable keeps in memory.
+// prefixes, announcements, peerings, BGP peers and origin changes. Its routes
+// table holds a copy of the whole routing table that internal/routetable
+// keeps in memory.
 package database
 
 import (