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

README.md

@@ -694,7 +694,10 @@ Key settings in more detail:
   and generator pages. Default: empty
 - `blocked_networks` — list of CIDR ranges to refuse for SSRF protection, added
   to the always-enforced built-in ranges (loopback, private, link-local, CGNAT,
-  benchmark, NAT64, and the like); an invalid CIDR aborts startup
+  benchmark, NAT64, and the like); an invalid CIDR aborts startup. An IPv6
+  address in the IPv4-mapped form (`::ffff:a.b.c.d`) is matched only as the IPv4
+  address it holds, so an IPv6 range such as `::/0` refuses neither it nor a
+  plain IPv4 address
 - `trusted_proxies` — list of CIDR ranges of the reverse proxies in front of
   pixa. `X-Forwarded-For` is believed only when the direct peer falls inside one
   of these ranges; the logged and login-recorded client address is then the