# Domain API Contract: Turn Navigation

## retreatTurn(encounter: Encounter): RetreatTurnSuccess | DomainError

### Input
- `encounter`: Encounter (combatants, activeIndex, roundNumber)

### Success Output
```
{
  encounter: Encounter  // Updated state
  events: DomainEvent[] // TurnRetreated, optionally RoundRetreated
}
```

### Error Cases

| Condition | Error Code | Message |
|-----------|------------|---------|
| combatants.length === 0 | `invalid-encounter` | Cannot retreat turn on an encounter with no combatants |
| roundNumber === 1 && activeIndex === 0 | `no-previous-turn` | Cannot retreat before the start of the encounter |

### Event Contracts

**TurnRetreated** (always emitted on success):
```
{
  type: "TurnRetreated"
  previousCombatantId: CombatantId  // Was active before retreat
  newCombatantId: CombatantId       // Now active after retreat
  roundNumber: number               // Round number after retreat
}
```

**RoundRetreated** (emitted when crossing round boundary):
```
{
  type: "RoundRetreated"
  newRoundNumber: number  // Decremented round number
}
```

**Emission order**: TurnRetreated first, then RoundRetreated (when applicable).

## UI Contract: TurnNavigation Component

### Props
- `encounter`: Encounter (current state)
- `onAdvanceTurn`: () => void
- `onRetreatTurn`: () => void

### Behavior
- Previous Turn button: calls onRetreatTurn; disabled when roundNumber === 1 && activeIndex === 0, or combatants.length === 0
- Next Turn button: calls onAdvanceTurn; disabled when combatants.length === 0
- Displays: round number and active combatant name