Write batches through one connection with a 1 GiB cache, in key order (closes #96)

Every write now uses one connection taken from the pool when the
database opens and used under the database lock. Its 1 GiB page cache
keeps the pages the route writes touch from one batch to the next. A
reading connection's cache is emptied whenever another connection has
written, so the 9 reading connections drop to 16 MiB each. The soft
and hard heap limits rise to 1.5 and 2 GiB so they do not shrink the
caches, and README "Memory" adds up the new budget. Batch writes sort
their rows by peer, then prefix. The benchmark also times the largest
prefix handler batch, 250,000 updates.

Model: opus-5-5

internal/database/slowquery.go

@@ -18,12 +18,12 @@ func logSlowQuery(logger *logger.Logger, query string, start time.Time) {
 	}
 }
 
-// exec wraps Exec with slow query logging
+// exec runs a statement on the writing connection, with slow query logging
 func (d *Database) exec(query string, args ...any) error {
 	start := time.Now()
 	defer logSlowQuery(d.logger, query, start)
 
-	_, err := d.db.ExecContext(context.Background(), query, args...)
+	_, err := d.writer.ExecContext(context.Background(), query, args...)
 
 	return err
 }