Generate the v2 -> v3 sync mapping for a Stream Activity Feeds app by sampling its live v2 activities and reactions. Use when setting up a Feeds v2 to v3 migration, when asked to build, review, or debug a v3 sync mapping, when someone asks 'what mapping do we need for our app?', or when a migrated activity lands in v3 with missing text, missing attachments, comments that are not comments, or bookmarks that went missing. Triggers on v3sync, v2 to v3, feeds migration, activity feeds migration, sync mapping, extra_context, reaction kinds, comments and bookmarks mapping. The same mapping drives both the live syncer and the bulk exporter.