AppendToIPSet now adds both IPv4 and IPv6 addresses for nodes,
matching Tailscale's FilterRule wire format where identity-based
aliases (tags, users, groups, autogroups) resolve to both address
families.
Address-based aliases (raw IPs, host names) are unchanged: they
resolve to exactly the literal prefix. The appendIfNodeHasIP helper
that incorrectly expanded address aliases to include the matching
node's other IPs is removed, fixing the RAW_IPV6_ADDR_EXPANSION
bug where a raw fd7a: IPv6 address would incorrectly include the
node's IPv4.
Updates #2180
ccade49742