Port the upstream IPv6 MAAD estimator: 128-bit prefixes, the nearly-full
test against 128 - pl, and upstream's default /23-/64 prefix range. The
q grid and full threshold match IPv4.

The pipeline now writes address_structure_stats rows for ip_version 6
scopes as well as 4. The product identity records both family configs
and bumps the MAAD contract version, so existing databases must be
rebuilt. netflow-db maad and validate_maad.py gain --ipv6, and IPv6
goldens are generated from the pinned oracle.

The web structure and spectrum routes take a validated ipVersion
parameter (default 4), and the dashboard spectrum card and file detail
page get an IPv4/IPv6 selector.