Idempotent person upsert keyed on the normalizer person_id (source_ref),
for the Phase-3 canonical importer. Re-import precedence (Resolved
decision #1): a non-blank existing field is never overwritten, blank
fields are filled from canonical, and provisional is monotonic — once a
human confirms a person (false) it never reverts to true. New
importer-created persons carry provisional=true; register persons false.
Maiden name is stored as a MAIDEN_NAME PersonNameAlias, matching the
existing findOrCreateByAlias behaviour.
Refs #669
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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