Migration 0263 added three columns beside the Apple Health `stats:*`
authority ladder. Only `aggregation_provenance` is ever read back: the
external-measurement reconciler selects it and refuses a lower-authority write
against a higher-authority canonical row, and the cumulative drain reads it
too. That column stays.
`aggregation_contributor_count` and `aggregation_selected_source_hash` are
written by three call sites and read by none. No select, no raw SQL, no export,
no backup, no API contract, no repair path. The import-time selection keeps its
deterministic tiebreak, which compares an in-memory map key computed per flush
rather than a value read back from the table.
Both are plain nullable scalars with no index, unique, check or foreign key, so
the drop is a catalog-only operation that touches no measured value.
