0081 — Measure synchronous_commit only; skip the batch-ack measurement

Context. The throughput wall is the per-message success/failure path: one single-row UPDATE … WHERE id=$1 AND lease_token=$2 via autocommit Pool.Exec, so every message pays its own commit and WAL fsync. The claim commits once per batch, so it already amortizes. An upcoming topology change (append-only log with cursors) will reshape the table layout.

Decision. Measure only synchronous_commit — whether COMMIT blocks on the fsync or returns early and lets the WAL writer flush asynchronously. It is the one portable lever: a global durability knob, blind to table layout, so the result survives the topology change.

Consequences. No commit-batching code is built. Rejected: measuring batch-ack — it was measure-only anyway, and the cursor model is its limit case (N messages recorded by one integer position write), so the bridge is understood without building it.