Ranks the SUBJECT axis: whose audit trail was read, within the period. It is the aggregate form of the question the meta plane exists to answer, and until now no surface could ask it.

A deliberately separate operation from getTopTenants, which ranks the READER tenant. An axis parameter that flipped which question a ranking answered would be the conflation this surface exists to remove, one layer up.

An empty ranking is a legible answer, not a fault. A read that addressed no single tenant contributes no row at all — absence is honest, and an empty-string bucket would collide with the reader-tenant total. Where every read is platform-scoped the ranking is correctly empty and the total is 0.

The period total counts the TARGET_TENANT population — the reads that addressed a tenant — and never the reader-tenant total, because a platform-scoped read appears in the second and not the first.