Implement the 023-clear-encounter feature that adds a clear encounter button with confirmation dialog to remove all combatants and reset round/turn counters, with the cleared state persisting across page refreshes

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

packages/domain/src/clear-encounter.ts

@@ -0,0 +1,33 @@
+import type { DomainEvent } from "./events.js";
+import type { DomainError, Encounter } from "./types.js";
+
+export interface ClearEncounterSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+export function clearEncounter(
+	encounter: Encounter,
+): ClearEncounterSuccess | DomainError {
+	if (encounter.combatants.length === 0) {
+		return {
+			kind: "domain-error",
+			code: "encounter-already-empty",
+			message: "Cannot clear an encounter that has no combatants",
+		};
+	}
+
+	return {
+		encounter: {
+			combatants: [],
+			activeIndex: 0,
+			roundNumber: 1,
+		},
+		events: [
+			{
+				type: "EncounterCleared",
+				combatantCount: encounter.combatants.length,
+			},
+		],
+	};
+}