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>

packages/domain/src/retreat-turn.ts

@@ -0,0 +1,59 @@
+import type { DomainEvent } from "./events.js";
+import type { DomainError, Encounter } from "./types.js";
+
+interface RetreatTurnSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+export function retreatTurn(
+	encounter: Encounter,
+): RetreatTurnSuccess | DomainError {
+	if (encounter.combatants.length === 0) {
+		return {
+			kind: "domain-error",
+			code: "invalid-encounter",
+			message: "Cannot retreat turn on an encounter with no combatants",
+		};
+	}
+
+	if (encounter.roundNumber === 1 && encounter.activeIndex === 0) {
+		return {
+			kind: "domain-error",
+			code: "no-previous-turn",
+			message: "Cannot retreat before the start of the encounter",
+		};
+	}
+
+	const previousIndex = encounter.activeIndex;
+	const wraps = previousIndex === 0;
+	const newIndex = wraps ? encounter.combatants.length - 1 : previousIndex - 1;
+	const newRoundNumber = wraps
+		? encounter.roundNumber - 1
+		: encounter.roundNumber;
+
+	const events: DomainEvent[] = [
+		{
+			type: "TurnRetreated",
+			previousCombatantId: encounter.combatants[previousIndex].id,
+			newCombatantId: encounter.combatants[newIndex].id,
+			roundNumber: newRoundNumber,
+		},
+	];
+
+	if (wraps) {
+		events.push({
+			type: "RoundRetreated",
+			newRoundNumber,
+		});
+	}
+
+	return {
+		encounter: {
+			combatants: encounter.combatants,
+			activeIndex: newIndex,
+			roundNumber: newRoundNumber,
+		},
+		events,
+	};
+}