prefer-ipv6 research DONE: the option never drove the v6 HAProxy bind ...

Answers the 2026-07-31 ruled question ("Research what those 6 charms do with
v6 VIPs first"). Measured by downloading each charm at its pinned channel for
amd64/ubuntu@22.04 and reading its templates and code -- not Charmhub's
config-yaml, which answers only whether the knob is declared.

FINDING: in BOTH template families the ':::port' frontend bind is gated on
ipv6_enabled = not is_ipv6_disabled(), a read of the kernel
net.ipv6.conf.all.disable_ipv6 sysctl -- MEASURED 0 on a live MAAS-deployed
jammy node in this DC. The v6 VIP is assigned by pacemaker on per-address
family detection. vault ships no haproxy template at all; its listener is the
hardcoded literal [::]:8200. prefer-ipv6 is a UNIT ADDRESS-FAMILY switch, and
the one value it sets that sounds like a bind address (haproxy_host) is
consumed by no template in any charm downloaded.

So option (b) -- drop the option from the six, keep their v6 VIP legs -- is
what the measurement supports. NOT TAKEN: the ruling is the operator's, the
overlay is untouched and R2 is unamended.

Also recorded: provider-bundle-check invariant 9's mechanism claim is wrong
and must be REPLACED (never deleted) when a ruling lands; a new and separate
question about the SEVEN charms that do declare the option; a full
option-name sweep of the dc0 deploy input (56 apps / 81 assignments / 20
schemas / 0 unresolvable) showing prefer-ipv6 on the six is the ONLY unknown
option, so nothing else is waiting for attempt 2; and a runbook defect --
the dc0 deploy block omits vr1-dc0-machines.yaml, which is NOT a no-op (it
carries ovn-chassis bridge-interface-mappings).

Read-only session: no cloud mutation, no overlay edit, no ruling adopted.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HvCyrwvYTTcDYnRErfMsNf