fix(auth): an IPv6 visitor counts by their /64

One household's line or one server gets a whole /64 of IPv6 addresses,
and could turn through them past every per-network limit (the second
reading's M5). networkOf now counts an IPv6 visitor by their /64
(networkKeyOf), an IPv4 visitor by their address as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_base_server/README.md

@@ -48,7 +48,9 @@ Endpoints, under the nickname `base` in a host's client (`client.modules.base.em
   connection's, or, for a connection from a proxy the operator trusts
   (`TRUSTED_PROXIES`), the last address of `X-Forwarded-For`, the one the proxy saw
   itself. Never the `Forwarded` header, which Caddy passes on as the visitor wrote it,
-  so never Serverpod's `remoteInfo`. Serverpod's own limit on anonymous accounts is off
+  so never Serverpod's `remoteInfo`. An IPv6 visitor counts by their /64
+  (`networkKeyOf`), which one line or server holds whole. Serverpod's own limit on
+  anonymous accounts is off
   (it counts by the connection's address, behind a proxy the proxy's for everyone); the
   endpoint counts them itself.
 - Refusals are `PasswordRefusedException`, `GitHubSignInRefusedException`,