Messaging family — first release

Added

Document	What it carries
`reference/messaging/pubsub.md`	four objects, five subscription types, at-least-once by default, ordering keys, dead-letter topics
`reference/messaging/cloud-tasks.md`	queue-as-policy, two target types, the three controls, and when to choose it over Pub/Sub
`reference/messaging/eventarc.md`	CloudEvents, the Cloud Audit Logs source, the destinations, Standard against Advanced
`reference/messaging/errors.md`	the six failures that are usually a contract misunderstanding
`guides/messaging/deliver-pubsub-to-cloud-run.md`	six steps, ending in a deliberate failure test
`guides/parity/messaging.md`	the one-to-three mapping, the partition question, and the consumers Pub/Sub deletes

Three Pub/Sub subscription types write into BigQuery, Bigtable or Cloud Storage with no code. Finding

the equivalent consumer in a source architecture and deleting it is the most commonly missed saving in a messaging migration.

Microsoft's own table maps Pub/Sub to three Azure services. That is the family's whole shape: one

Google Cloud product against two or three elsewhere.

The Pub/Sub overview states no default message retention, so none is quoted; the error catalogue says to

read it from the subscription.

Pub/Sub Lite is referenced only as a separate documentation set. Nothing about its status is claimed.
The Cloud Tasks overview does not compare itself to Pub/Sub; the choose-between rule in

reference/messaging/cloud-tasks.md §4 is labelled as the hub's reading.

Cloud Scheduler and Workflows, both named in the parity table and not documented.
Pub/Sub schema evolution, snapshots and seek (replay).
Subscription filtering.
Service-to-service authentication details for push endpoints beyond the invoker role.
Eventarc Advanced pipeline configuration.