Evaluate formal NSDI/GDA discovery metadata construct

Structured subgroup of fields to house boolean and tier classification; would replace keyword overloading for NGDA too

Migrate NGDA tagging out of keyword field into new discovery construct

Depends on NSDI construct decision above

Start by resolving the formal NSDI/GDA discovery-construct decision described in the issue, then trace the existing NGDA tagging that currently uses the keyword field. Done means the construct is defined and NGDA tagging is migrated to it without relying on keyword overloading.

Written by the indexing model from the issue text.