merge_canonical_entities

Merge two or more canonical registry entities into one surviving canonical: the fix for twin canonicals created across separate canonicalization passes (e.g. `persona_editor` + `persona_editor_2` with every instance registered under both). In one atomic flush: repoints every cross-edge from `registry/{loser}` to `registry/{keep}` (a repoint that would duplicate an existing edge, the double-parented instance case, drops the redundant edge instead, preserving any `alias` sanction), repoints registry-internal edges (self-loops and duplicates drop), unions each loser's description/tags/properties into the keeper's GAPS (the keeper always wins where both define a value), then deletes the losers. All canonicals must share one entity type. Preview the full plan with `dry_run: true`.