Get a trader grade proven visible at a past instant

Reads one trader's recorded grade at as_of from prospective visibility evidence. Before the first recorded observation, after deletion, or during a grade-transition gap, status is unknown and grade is null; a recorded ungraded trader instead has status ungraded. available_from is the first proven observation for this trader, not a claim that earlier grades can be reconstructed. observed_at is when the grade writer began the transition and published_by is the later confirmation that the row was committed, not an exact commit time. Older observations have null model_version, model_build_sha and source_observed_by; do not substitute a current grade or a dated ranking row. The requested event time, publication bounds and model provenance are separate. Use a grade only when status is graded. This is additive; the current-grade trader and leaderboard operations do not change.