Implement the 002-add-combatant feature that adds the possibility to add new combatants to an encounter

packages/domain/src/add-combatant.ts

@@ -0,0 +1,50 @@
+import type { DomainEvent } from "./events.js";
+import type { CombatantId, DomainError, Encounter } from "./types.js";
+
+export interface AddCombatantSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+/**
+ * Pure function that adds a combatant to the end of an encounter's list.
+ *
+ * FR-001: Accepts an Encounter, CombatantId, and name; returns next state + events.
+ * FR-002: Appends new combatant to end of combatants list.
+ * FR-004: Rejects empty/whitespace-only names with DomainError.
+ * FR-005: Does not alter activeIndex or roundNumber.
+ * FR-006: Events returned as values, not dispatched via side effects.
+ */
+export function addCombatant(
+	encounter: Encounter,
+	id: CombatantId,
+	name: string,
+): AddCombatantSuccess | DomainError {
+	const trimmed = name.trim();
+
+	if (trimmed === "") {
+		return {
+			kind: "domain-error",
+			code: "invalid-name",
+			message: "Combatant name must not be empty",
+		};
+	}
+
+	const position = encounter.combatants.length;
+
+	return {
+		encounter: {
+			combatants: [...encounter.combatants, { id, name: trimmed }],
+			activeIndex: encounter.activeIndex,
+			roundNumber: encounter.roundNumber,
+		},
+		events: [
+			{
+				type: "CombatantAdded",
+				combatantId: id,
+				name: trimmed,
+				position,
+			},
+		],
+	};
+}