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Sibling to BuildForTwoPickAsync. Routes through IDeckRepository.GetDeck to pull the viewer's deck #1 for the requested format (avoiding the viewer-graph nav-ref auto-load pitfall — DeckCard.Card silently ships card_id=0 via the default include path). Throws if the viewer has no deck for the format. Cosmetics fall back to DefaultLoadoutConfig defaults when unequipped, same shape as TK2. Used by RankBattleController in a later task to build self-context for /ai_<fmt>_rank_battle/start and to pair-up under /<fmt>_rank_battle/do_matching. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>