Widen dedup match to 4-char grid prefix so an upload with a longer grid
(e.g. FN42aa25) finds the existing FN42 contact. Within a match, classify
as a refinement when the upload strictly extends the existing grid or
fills a missing mode; as a contradiction (still skipped) when grids or
modes genuinely disagree. Refinements flow through a new preview bucket,
update the existing row in place, and re-enqueue enrichment when the
position changed.
