The runtime for shared topics:
- pubsub_repo: fan_out_publish gains `AND t.shared = FALSE` (a shared
trigger never fires on a per-app publish); new fan_out_shared_publish
matches `shared = true` pubsub triggers on the resolved owning group,
each delivery stamped with the writer app_id (M2 model).
- GroupPubsubService trait (shared) + GroupPubsubServiceImpl: resolve the
owning group (kind='topic') from cx.app_id's chain, require editor+
(GroupPubsubPublish, fails closed for anon), size-cap, fan out.
- SDK: `pubsub::shared_topic("name")` -> GroupTopicHandle with
`.publish(subtopic, msg)` / `.publish(msg)`; wired through Services +
the picloud binary (reuses the one PubsubRepo).
- authz: Capability::GroupPubsubPublish(GroupId), editor+ / script:write.
The owning-group chain walk is the isolation boundary; a sibling-subtree
app never resolves the topic.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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