engine: deliverPendingToNode keeps sending higher seqs after a lower-seq send fails — monotonic-ordering contract not enforced

[willwashburn]

Severity: minor · packages/engine/src/engine/delivery.ts:894-919

for (const row of rows) {           // ordered asc(seq)
  ...
  const sent = await registry.sendToProvider(...);
  if (sent) deliveredIds.push(row.delivery.id);  // no break on a failed lower seq
}
await markDeliveriesDelivered(db, workspaceId, deliveredIds);

The replay loop continues to the next row even when an earlier (lower-seq) send returned false, so seq N+1 can land on the wire while seq N stayed queued — despite the comment at delivery.ts:762-766 stating these rows form "an ordered, seq-monotonic stream" the broker gates on.

Impact is low in practice (a mid-stream failure usually means a dead socket, so subsequent sends also fail, and queued rows are re-replayed next pass) — but the stated contract isn't enforced; a partial-failure transport creates a wire gap the broker must buffer around.

Fix: break on the first failed send within an agent's stream (per-agent, so one agent's failure doesn't block others).

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Found during a multi-agent reliability review of the relay tool/protocol.