Single-node ETS Pushed Authorization Request store.
This is a development/default store. Clustered deployments should provide a
AttestoPhoenix.PARStore backed by shared storage.
The table is bounded by :par_max_entries and :par_max_record_bytes from
AttestoPhoenix.Config. Admission is serialized in the table owner: expired
rows are pruned first, a live random-reference collision is rejected, and a
new live row is never evicted to make room.