b0358cf3c8
Post-rebase consolidation over the merged C7/C16/C10 work: - _ensure_dm_conversation now records workspace ownership via _remember_channel_team and bounds _dm_conversation_cache (cap 5000) - module-level _slack_dm_cache (C7 standalone path) bounded oldest-first - _user_is_bot_cache (C10) bounded with _trim_oldest_dict_entries - caption-mode contract tests updated for the C7+C8 merged media path