Refuse IPv4-mapped prefixes with an error that says so

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/model/addr.go

@@ -84,7 +84,11 @@ func ParseAddress(s string) (Address, error) {
 		if err != nil {
 			return Address{}, fmt.Errorf("%q is not an address or prefix", s)
 		}
-		p = netip.PrefixFrom(p.Addr().Unmap(), p.Bits())
+		if p.Addr().Is4In6() {
+			// Unmapping would leave an IPv4 address with IPv6 bits
+			// (::ffff:0:0/96 as 0.0.0.0/96).
+			return Address{}, fmt.Errorf("%q is an IPv4-mapped prefix, which can't be used: write IPv4 prefixes as IPv4 (10.0.0.0/8)", s)
+		}
 		if p.Masked() != p {
 			return Address{}, fmt.Errorf("%q has host bits set; the prefix is %s", s, p.Masked())
 		}