Implements the class-selection and card-pick turns for the Take Two arena draft:
- ChooseClassAsync validates class is offered, locks ClassId, generates first pick set via pool
- ChooseCardAsync appends the two picked cards, advances SelectTurn 1–15, completes draft at turn 15
- 6 new tests covering happy paths and all error codes (class_not_offered, invalid_state, invalid_selection)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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