Provenance: content-match:<kind> or alias:<reason> for identity,
split:<confidence> / merge:<confidence> for those, and
successor:<confidence> for replaced.
Reasons are provenance, not a schema discriminator: an incoming lineage's
explicit relation wins on replay. For aliases coming from an identity map
(which has no relation field), successor: means replaced; every other
reason means identity.
OptionalsharesFor a split, each head key's share of the pieces' total volume, in the
order of head; for a merge, each base key's share, in the order
of base. Present only when every piece carried a proved volume
(the claim's confidence was not extent). The largest-share rekey policy
reads it; without it only copy-to-all and orphan-on-split are safe.
One lineage entry.
baseandheadare key lists; exactly one of them has more than one key, and only forsplit(k heads) ormerge(k bases).