0327 — Commit's per-exception writes are batched; RecordException stays one message at a time

Context. Write-path round trips were audited per function rather than batched wholesale. Two candidates: the loop in Commit/PartialCommit that records exception and terminal rows (it runs after every consumerFunc call has already finished, inside an already-open transaction), and RecordException*/Record*’s one-message-at-a-time calls.

Decision. The Commit/PartialCommit loop collapsed from 2 sequential round trips per recorded message to one pgx.Batch. RecordException*/Record* were deliberately left unbatched.

Consequences. Measured: recording N exceptions went from 3.16ms at N=1 to 9.3ms at N=1000 — total wall-clock grew ~3x while N grew 1000x. Rejected: batching RecordException*/Record* — it would defer the durable write past multiple consumerFunc calls, trading a durability/idempotency guarantee for a performance win nothing asked for.