Function resolveAttachedLeader

The roster's leader entry attached to bodyguardUnitId, if any. Import stores the inferred (always-provisional) attachment on the leader's RosterUnit, pointing down to its bodyguard via leader_attachment.bodyguard_ref. Selection UIs start from the body unit, so this scans for the leader whose bodyguard_ref.id matches. Returns undefined when no leader in the roster is attached to that unit (the common case — attachments are optional at game start).