Same pattern as rank-battle: DoMatching stashes context; Finish takes
it and records history + played-together. Opponent identity is left
as placeholder fields until the resolver carries it through.
Test seeds an active ViewerArenaTwoPickRun so RecordBattleResultAsync
does not throw no_active_run during the e2e flow.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>