code_family_index(families) builds a code -> family key map once so a
restamp run of ~1.15M chunks doesn't rescan every family per code.
anchor_metadata gains an optional code_index kwarg that looks family
membership up through it instead.
Ruling A5: families with fewer than two codes are excluded from the
index and from the default unindexed scan — the derived registry is
~17,187 keys and all but a handful are single-code stem families,
where a 'families' anchor would just echo the code already in
'codes'. Only real groupings (today: ACP, CCM, PCM, TCM, APCM) get a
families anchor.
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