fix: fold a role mailbox's case, and test the half that was not tested

"Abuse@Host.Invalid" and "abuse@host.invalid" produced two destinations and
two mails to one desk, which is the duplicate-mail behaviour the grouping
exists to prevent.

The root cause is the test, not the grouping. _group_key folded the domain
correctly, but the test written for it used a lowercase local part in both
of its inputs, so it varied only the half that worked and passed while the
other half was never exercised. A test that varies one half of its input
proves nothing about the other.

The local part now folds for the RFC 2142 role mailboxes. That RFC mandates
those names and requires them matched case-insensitively, so no host runs
"Abuse@" and "abuse@" as different desks and treating them as two costs a
duplicate mail with nothing on the other side of the trade. This reverses
the earlier decision only for the standardised names: any other local part
is still left as published, because for a named mailbox folding could merge
two desks a host genuinely distinguishes, and a dropped desk is worse than a
duplicate. Each half now folds on the strength of its own standard.

Both folds are mutation-checked and killed by distinct tests, so neither
masks the other, and the role fold is checked across all five role names.

Also pins two behaviours that held only by luck: the caller's contacts are
not modified, and a destination's ioc list is not aliased to the contact's.
Both are true today because the grouping starts a fresh list, and nothing
would have caught an implementation that reused the contact's.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xj1ayFRSUQ2u7cwb3S4axE

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