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/data-model.md

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+# Data Model: Turn Navigation
+
+## Existing Entities (unchanged)
+
+### Encounter
+- `combatants`: readonly array of Combatant
+- `activeIndex`: number (0-based index into combatants)
+- `roundNumber`: positive integer (>= 1)
+
+### Combatant
+- `id`: CombatantId (branded string)
+- `name`: string
+- `initiative?`: number
+- `maxHp?`: number
+- `currentHp?`: number
+
+## New Domain Events
+
+### TurnRetreated
+Emitted on every successful RetreatTurn operation.
+
+| Field | Type | Description |
+|-------|------|-------------|
+| type | `"TurnRetreated"` (literal) | Discriminant for event union |
+| previousCombatantId | CombatantId | The combatant whose turn was active before retreat |
+| newCombatantId | CombatantId | The combatant who is now active after retreat |
+| roundNumber | number | The round number after the retreat |
+
+### RoundRetreated
+Emitted when RetreatTurn crosses a round boundary (activeIndex wraps from 0 to last combatant).
+
+| Field | Type | Description |
+|-------|------|-------------|
+| type | `"RoundRetreated"` (literal) | Discriminant for event union |
+| newRoundNumber | number | The round number after decrementing |
+
+## State Transitions
+
+### RetreatTurn
+
+**Input**: Encounter
+**Output**: `{ encounter: Encounter, events: DomainEvent[] }` | `DomainError`
+
+**Rules**:
+1. If `combatants.length === 0` -> DomainError("invalid-encounter")
+2. If `roundNumber === 1 && activeIndex === 0` -> DomainError("no-previous-turn")
+3. If `activeIndex > 0`: newIndex = activeIndex - 1, roundNumber unchanged
+4. If `activeIndex === 0`: newIndex = combatants.length - 1, roundNumber - 1
+
+**Events emitted**:
+- Always: TurnRetreated
+- On round boundary crossing (rule 4): TurnRetreated then RoundRetreated (order matters)
+
+## Validation Rules
+
+- RetreatTurn MUST NOT produce roundNumber < 1
+- RetreatTurn MUST NOT produce activeIndex < 0 or >= combatants.length
+- RetreatTurn is a pure function: identical input produces identical output