net: add two seed nodes and reserve names for three more

node6.dragonx.is (13.140.58.251) and node7.dragonx.is (5.104.83.100)
are new full nodes in regions the existing five did not cover. Both go
into the compiled-in fixed-seed list and into the DRAGONX -addnode set.

node8 through node10 are reserved names with no DNS records yet. A
hostname that does not resolve is harmless on this path --
ThreadOpenAddedConnections simply fails to open the connection and
retries on its normal cycle -- and reserving the names in the binary
means a future seed can be brought into the default addnode set by
creating a single DNS record, with no release and no waiting for users
to upgrade. seed.dragonx.is already gives the DNS-seed path that
property; this extends it to the addnode path.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FU87LdsJZiZkfq1eXubpeo

contrib/seeds/nodes_main.txt

@@ -15,3 +15,9 @@
 
 # node5.dragonx.is
 176.126.87.241:21768
+
+# node6.dragonx.is
+13.140.58.251:21768
+
+# node7.dragonx.is
+5.104.83.100:21768