DOCFIX-164: D-101 IPv6 family-matrix overlay, researched against real charm source (gap #13) ...

overlays/dc-dc-ipv6-family-matrix.yaml + docs/dc-dc-ipv6-charm-research.md.
Fetched real charm config/source (not memory): prefer-ipv6 is a real,
shared charms.openstack option, confirmed genuinely additive dual-stack
(not either/or) via charm-nova-cloud-controller's actual "Dual Stack VIPs"
commit; ceph-mon's own prefer-ipv6 is a straight switch, correct for the
ULA-only planes; OVN has no such option at all (confirmed by reading its
full config.yaml) and needs no overlay entry; vault's cert code has no
v4/v6 distinction, confirming D-109's SAN requirement is code-supported.

Found and deliberately did not paper over a real open risk: Octavia's
lb-mgmt-net has two real, still-referenced upstream IPv6 bug reports.
Excluded from the overlay pending an explicit operator decision.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QrcJx8TUar7pYAvpGJw57A