ADR-019/020: second review round — handle surfaces, enqueue semantics, payload bridge + mechanical fixes

ADR-019: mechanism-handle traits pinned (Queue/StreamHandle/Outbox/
Lock/JobHandle), Job/Schedule struct shapes, worker_id claimant
identity, core StopToken — closes the 'pin at implementation' residue
before ADR-017's amend-in-place window terminates at first release.

ADR-020: delay-wins-over-run_at precedence (honker parity), relative
expires, scheduler-fired stamps from derived queue defaults, serde_json
byte-level payload encoding.

Mechanical: overview/engine-postgres ADR tables completed through 020,
core-contract Errors section re-framed to class-1, ADR-014 sketch
annotated with publish_with_key_tx, ADR-015 status dated, reserved-
namespace kinds list normalized to six kinds, redrive gate wording
aligned, broken link fixed, StopToken deferral superseded in ADR-009.

docs/architecture/decisions/014-outbox-tx-enqueue.md

@@ -54,6 +54,9 @@ One method, added to the `TxHandle` trait:
 trait TxHandle {
     enqueue_tx(queue, opts, payload) -> job_id
     publish_tx(stream, payload) -> offset
+    publish_with_key_tx(stream, key, payload) -> offset   // ADR-015,
+                                        // same day — not in this
+                                        // ADR's original sketch
     notify_tx(channel, payload)
     save_offset_tx(stream, consumer, offset)
     outbox_enqueue_tx(outbox, opts, payload) -> job_id   // the addition