Let the publisher accept transport headers and surface non-bq headers to
the handler through BabelHandler.headersOf, so the OTel wrapper can link
send and process spans across the hop (ADR-0028).

Accept an optional DeadLetterObserver and call it exactly once after each
confirmed DLQ write (and, on ASB, after an SDK-initiated native
dead-letter); observer failures never change the settle outcome
(ADR-0039).

Fall back from a blank meta.queue to the consumed queue only for the
dead-letter observer event, using the core isBlankAscii check; the DLQ
record's dead_letter.original_queue is unchanged, so the wire is too.