Implement the 012-turn-navigation feature that adds a RetreatTurn domain operation and relocates turn controls to a navigation bar at the top of the encounter tracker

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

specs/012-turn-navigation/contracts/domain-api.md

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+# 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