gh-107205: Align IPv4-mapped IPv6 networks with the embedded IPv4 properties

[rajorin]

IPv4-mapped addresses already take is_private and is_global from the embedded IPv4 address, but IPv6Network still treated every subnet of ::ffff:0:0/96 as private. ip_network('::ffff:100.64.0.0') was therefore private while ip_address of the same address was neither private nor global.

Subnets strictly inside the mapped range now follow the embedded IPv4 network. ::ffff:0:0/96 itself stays an IPv6 special-purpose block.

#107205

• Issue: ipaddress: ip_address IPv6 has wrong properties for Shared Address Space (100.64.0.0) #107205