Refuse local-use NAT64 addresses, and IPv4-compatible ones in blocked_networks (closes #299)

The check that keeps pixa from fetching internal addresses let through
64:ff9b:1::/48, the local-use NAT64 range, and matched an address in the
IPv4-compatible form (::/96) against blocked_networks only as IPv6, so
::203.0.113.5 passed with 203.0.113.0/24 listed. The local-use NAT64
range is now blocked next to 64:ff9b::/96, and an address in ::/96 is
refused when blocked_networks holds it as written or the IPv4 address it
holds. The mapped form stays matched as IPv4 only, as Go keeps plain
IPv4 addresses in that form; README.md says so.

Model: opus-5-5

TODO.md

@@ -30,6 +30,18 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-16 P1 security bug: the check that keeps pixa from fetching internal
+  addresses refuses the local-use NAT64 range (`64:ff9b:1::/48`, RFC 8215) as it
+  refuses `64:ff9b::/96`, and refuses an IPv6 address in the IPv4-compatible
+  form (`::/96`) when `blocked_networks` contains either the address as written
+  or the IPv4 address it holds (closes #299). A source host such as
+  `[64:ff9b:1::a00:1]` was fetched, which on a network that runs NAT64 on that
+  range reaches the IPv4 address it holds, here `10.0.0.1`; and with
+  `203.0.113.0/24` in `blocked_networks`, `[::ffff:203.0.113.5]` was refused but
+  `[::203.0.113.5]` was fetched. Both now answer 403. The mapped form
+  (`::ffff:a.b.c.d`) is still matched only as the IPv4 address it holds: Go
+  keeps a plain IPv4 address in that form, and an IPv6 entry such as `::/0` must
+  not refuse every IPv4 address.
 - 2026-10-16 P1 security bug: the check that keeps pixa from fetching internal
   addresses judges an IPv6 address in the IPv4-compatible form (`::/96`), such
   as `::127.0.0.1` or `::10.0.0.1`, by the IPv4 address in its last four bytes