…s into job (1.20.0)

EnvelopeCodec.make gains a keyword-only meta parameter; a collision with a core
or forbidden meta key raises EnvelopeKeyError. encode now raises on forbidden
keys and a top-level urn instead of dropping them. decode folds urn into job
(blank ASCII job falls back to urn; job wins over urn with a warning). The
conformance runner implements the 1.4.0 fields plus extension_write and
encode_reject, with a runner self-test.

… and Pulsar paths

encode now rejects forbidden keys and a top-level urn, so paths that parsed a
raw body and re-encoded it would raise on legacy bodies. Decode first (drops
and warns, folds urn into job, keeps unknown keys). Adds regression tests for
the runtime, SQS, Pulsar, Kafka and Azure Service Bus paths.

…g-only urn()

- make(): build the meta block as Dict[str, Any] so mypy passes without ignores
- raw RELEASE re-publish now normalises any body carrying a urn alias
- urn()/accepts() require a string job; falsy non-string job counts as blank for the urn fold
- CHANGELOG entries moved under [Unreleased]; has_forbidden_keys docstring notes urn exclusion