feat(shared-topics): GroupPubsubService + shared publish fan-out + SDK handle (D2)

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>

crates/manager-core/src/pubsub_service.rs

@@ -386,6 +386,17 @@ mod tests {
             self.written.lock().unwrap().extend(staged);
             Ok(u32::try_from(n).unwrap_or(u32::MAX))
         }
+
+        async fn fan_out_shared_publish(
+            &self,
+            _ctx: PublishCtx,
+            _owning_group: picloud_shared::GroupId,
+            _topic: &str,
+            _event_payload: serde_json::Value,
+        ) -> Result<u32, PubsubRepoError> {
+            // The per-app PubsubService tests don't exercise the shared path.
+            Ok(0)
+        }
     }
 
     #[derive(Default)]