ADR-012: billable events carry AR dimensions in metadata

ADR-012 now says where the AR-specific billing dimensions live on a billable event: ar_namespace_id rides in the metadata object of the billable_usage payload, next to feature_qualified_name. The context-schemas bullet no longer calls the namespace UUID a billing join key, since that copy is for analytics joins.

Reason: the billing pipeline reads only the top-level billable_usage fields plus the whole metadata blob, and never sees the Snowplow custom contexts. So an ar_namespace_id sent only in artifact_registry_context would not reach billing. Confirmed with Fulfillment: Utilization in this note.